Skip submission tests on *nix

src/Compilers/CSharp/Test/Symbol/Compilation/CompilationAPITests.cs

@@ -1383,7 +1383,7 @@ class A
                 Diagnostic(ErrorCode.WRN_MainIgnored, "Main").WithArguments("A.Main()").WithLocation(4, 17));
         }
 
-        [ClrOnlyFact(ClrOnlyReason.Unknown)]
+        [ClrOnlyFact(ClrOnlyReason.Submission)]
         public void GetEntryPoint_Submission()
         {
             var source = @"1 + 1";
@@ -1402,7 +1402,7 @@ public void GetEntryPoint_Submission()
             entryPoint.Diagnostics.Verify();
         }
 
-        [ClrOnlyFact(ClrOnlyReason.Unknown)]
+        [ClrOnlyFact(ClrOnlyReason.Submission)]
         public void GetEntryPoint_Submission_MainIgnored()
         {
             var source = @"

src/Compilers/CSharp/Test/Symbol/Compilation/GetUnusedImportDirectivesTests.cs

@@ -359,7 +359,7 @@ public void InfoAndWarnAsError()
                 Diagnostic(ErrorCode.HDN_UnusedUsingDirective, "using System;").WithWarningAsError(false));
         }
 
-        [Fact]
+        [ClrOnlyFact(ClrOnlyReason.Submission)]
         public void UnusedUsingInteractive()
         {
             var tree = Parse("using System;", options: TestOptions.Interactive);

src/Compilers/CSharp/Test/Symbol/Symbols/ExtensionMethodTests.cs

@@ -3685,7 +3685,7 @@ class C
             compilation.VerifyDiagnostics();
         }
 
-        [ClrOnlyFact]
+        [ClrOnlyFact(ClrOnlyReason.Submission)]
         public void InteractiveExtensionMethods()
         {
             var parseOptions = TestOptions.Interactive;

src/Compilers/CSharp/Test/Symbol/Symbols/Source/ExternAliasTests.cs

@@ -98,7 +98,7 @@ public void ExternAliasInScript()
             comp.VerifyDiagnostics();
         }
 
-        [ClrOnlyFact(ClrOnlyReason.Unknown)]
+        [ClrOnlyFact(ClrOnlyReason.Submission)]
         public void ExternAliasInInteractive_Error()
         {
             var src = "extern alias Bar;";

src/Compilers/VisualBasic/Test/Semantic/Semantics/GetUnusedImportDirectivesTests.vb

@@ -301,7 +301,7 @@ Imports System
             CreateCompilationWithMscorlib(source, parseOptions:=New VisualBasicParseOptions(documentationMode:=DocumentationMode.Diagnose)).AssertTheseDiagnostics(<errors></errors>, suppressInfos:=False)
         End Sub
 
-        <Fact()>
+        <ClrOnlyFact(ClrOnlyReason.Submission)>
         Public Sub UnusedImportInteractive()
             Dim tree = Parse("Imports System", options:=TestOptions.Interactive)
             Dim compilation = VisualBasicCompilation.CreateSubmission("sub1", tree, {MscorlibRef_v4_0_30316_17626})

src/Test/Utilities/Desktop/ConditionalFactAttribute.cs

@@ -35,6 +35,9 @@ public enum ClrOnlyReason
 
         // Can't sign. 
         Signing,
+
+        // Can't find System.Object when compiling a submission (not understood).
+        Submission,
     }
 
     public class ClrOnlyFact : FactAttribute