Just use an empty sequence to indicate no signature help was provided.

src/EditorFeatures/CSharpTest/SignatureHelp/InvocationExpressionSignatureHelpProviderTests.cs

@@ -6,6 +6,7 @@
 using Roslyn.Test.Utilities;
 using Xunit;
 using Microsoft.CodeAnalysis.CSharp;
+using System.Linq;
 
 namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.SignatureHelp
 {
@@ -918,7 +919,7 @@ void Foo(int a)
         Foo(/*,$$*/);
     }
 }";
-            Test(markup, usePreviousCharAsTrigger: true, expectSuccess: false);
+            Test(markup, Enumerable.Empty<SignatureHelpTestItem>(), usePreviousCharAsTrigger: true);
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)]
@@ -933,7 +934,7 @@ void Foo(int a)
             );
     }
 }";
-            Test(markup, usePreviousCharAsTrigger: true, expectSuccess: false);
+            Test(markup, Enumerable.Empty<SignatureHelpTestItem>(), usePreviousCharAsTrigger: true);
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)]
@@ -947,7 +948,7 @@ void Foo(int a)
         Foo("",$$"");
     }
 }";
-            Test(markup, usePreviousCharAsTrigger: true, expectSuccess: false);
+            Test(markup, Enumerable.Empty<SignatureHelpTestItem>(), usePreviousCharAsTrigger: true);
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)]

src/EditorFeatures/Test/SignatureHelp/AbstractSignatureHelpProviderTests.cs

@@ -46,17 +46,16 @@ public void SetFixture(TWorkspaceFixture workspaceFixture)
             IEnumerable<SignatureHelpTestItem> expectedOrderedItemsOrNull = null,
             bool usePreviousCharAsTrigger = false,
             SourceCodeKind? sourceCodeKind = null,
-            bool experimental = false,
-            bool expectSuccess = true)
+            bool experimental = false)
         {
             if (sourceCodeKind.HasValue)
             {
-                TestSignatureHelpWorker(markup, sourceCodeKind.Value, experimental, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, expectSuccess);
+                TestSignatureHelpWorker(markup, sourceCodeKind.Value, experimental, expectedOrderedItemsOrNull, usePreviousCharAsTrigger);
             }
             else
             {
-                TestSignatureHelpWorker(markup, SourceCodeKind.Regular, experimental, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, expectSuccess);
-                TestSignatureHelpWorker(markup, SourceCodeKind.Script, experimental, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, expectSuccess);
+                TestSignatureHelpWorker(markup, SourceCodeKind.Regular, experimental, expectedOrderedItemsOrNull, usePreviousCharAsTrigger);
+                TestSignatureHelpWorker(markup, SourceCodeKind.Script, experimental, expectedOrderedItemsOrNull, usePreviousCharAsTrigger);
             }
         }
 
@@ -65,8 +64,7 @@ public void SetFixture(TWorkspaceFixture workspaceFixture)
             SourceCodeKind sourceCodeKind,
             bool experimental,
             IEnumerable<SignatureHelpTestItem> expectedOrderedItemsOrNull = null,
-            bool usePreviousCharAsTrigger = false,
-            bool expectSuccess = true)
+            bool usePreviousCharAsTrigger = false)
         {
             markupWithPositionAndOptSpan = markupWithPositionAndOptSpan.NormalizeLineEndings();
 
@@ -94,7 +92,7 @@ public void SetFixture(TWorkspaceFixture workspaceFixture)
                 document1 = document1.Project.WithParseOptions(parseOptions).GetDocument(document1.Id);
             }
 
-            TestSignatureHelpWorkerShared(code, cursorPosition, sourceCodeKind, document1, textSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, expectSuccess);
+            TestSignatureHelpWorkerShared(code, cursorPosition, sourceCodeKind, document1, textSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger);
 
             // speculative semantic model
             if (CanUseSpeculativeSemanticModel(document1, cursorPosition))
@@ -105,7 +103,7 @@ public void SetFixture(TWorkspaceFixture workspaceFixture)
                     document2 = document2.Project.WithParseOptions(parseOptions).GetDocument(document2.Id);
                 }
 
-                TestSignatureHelpWorkerShared(code, cursorPosition, sourceCodeKind, document2, textSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, expectSuccess);
+                TestSignatureHelpWorkerShared(code, cursorPosition, sourceCodeKind, document2, textSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger);
             }
         }
 
@@ -381,8 +379,7 @@ protected void VerifyItemWithReferenceWorker(string xmlString, IEnumerable<Signa
             Document document,
             TextSpan? textSpan,
             IEnumerable<SignatureHelpTestItem> expectedOrderedItemsOrNull = null,
-            bool usePreviousCharAsTrigger = false,
-            bool expectSuccess = true)
+            bool usePreviousCharAsTrigger = false)
         {
             var signatureHelpProvider = CreateSignatureHelpProvider();
             var triggerInfo = new SignatureHelpTriggerInfo(SignatureHelpTriggerReason.InvokeSignatureHelpCommand);
@@ -401,12 +398,6 @@ protected void VerifyItemWithReferenceWorker(string xmlString, IEnumerable<Signa
 
             var items = signatureHelpProvider.GetItemsAsync(document, cursorPosition, triggerInfo, CancellationToken.None).Result;
 
-            if (!expectSuccess)
-            {
-                AssertEx.Null(items);
-                return;
-            }
-
             // If we're expecting 0 items, then there's no need to compare them
             if ((expectedOrderedItemsOrNull == null || !expectedOrderedItemsOrNull.Any()) && items == null)
             {

src/EditorFeatures/VisualBasicTest/SignatureHelp/AbstractVisualBasicSignatureHelpProviderTests.vb

@@ -11,11 +11,11 @@ Namespace Microsoft.CodeAnalysis.Editor.VisualBasic.UnitTests.SignatureHelp
 
         ' We want to skip script testing in all VB stuff for now.
 
-        Protected Overrides Sub Test(markupWithPositionAndOptSpan As String, Optional expectedOrderedItemsOrNull As IEnumerable(Of SignatureHelpTestItem) = Nothing, Optional usePreviousCharAsTrigger As Boolean = False, Optional sourceCodeKind As Microsoft.CodeAnalysis.SourceCodeKind? = Nothing, Optional experimental As Boolean = False, Optional expectSuccess As Boolean = True)
+        Protected Overrides Sub Test(markupWithPositionAndOptSpan As String, Optional expectedOrderedItemsOrNull As IEnumerable(Of SignatureHelpTestItem) = Nothing, Optional usePreviousCharAsTrigger As Boolean = False, Optional sourceCodeKind As Microsoft.CodeAnalysis.SourceCodeKind? = Nothing, Optional experimental As Boolean = False)
             If (sourceCodeKind.HasValue) Then
-                MyBase.Test(markupWithPositionAndOptSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, sourceCodeKind, experimental, expectSuccess)
+                MyBase.Test(markupWithPositionAndOptSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, sourceCodeKind, experimental)
             Else
-                MyBase.Test(markupWithPositionAndOptSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, Microsoft.CodeAnalysis.SourceCodeKind.Regular, experimental, expectSuccess)
+                MyBase.Test(markupWithPositionAndOptSpan, expectedOrderedItemsOrNull, usePreviousCharAsTrigger, Microsoft.CodeAnalysis.SourceCodeKind.Regular, experimental)
             End If
         End Sub
 

src/EditorFeatures/VisualBasicTest/SignatureHelp/InvocationExpressionSignatureHelpProviderTests.vb

@@ -983,7 +983,7 @@ Class C
     End Sub
 End Class
 "
-            Test(markup, usePreviousCharAsTrigger:=True, expectSuccess:=False)
+            Test(markup, Enumerable.Empty(Of SignatureHelpTestItem)(), usePreviousCharAsTrigger:=True)
         End Sub
 
         <Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)>
@@ -995,7 +995,7 @@ Class C
     End Sub
 End Class
 "
-            Test(markup, usePreviousCharAsTrigger:=True, expectSuccess:=False)
+            Test(markup, Enumerable.Empty(Of SignatureHelpTestItem)(), usePreviousCharAsTrigger:=True)
         End Sub
 
         <Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)>