Remove some gratuitious uses of GetOpenTextContainer()

The only remaining uses now are ones actually testing the core workspace.

src/EditorFeatures/CSharpTest/ExtractMethod/MiscTests.cs

@@ -131,7 +131,6 @@ public void TestExtractMethodCommandHandlerErrorMessage()
 
             using var workspace = TestWorkspace.CreateCSharp(markupCode);
             var testDocument = workspace.Documents.Single();
-            var container = testDocument.GetOpenTextContainer();
 
             var view = testDocument.GetTextView();
             view.Selection.Select(new SnapshotSpan(

src/EditorFeatures/TestUtilities/LanguageServer/AbstractLanguageServerProtocolTests.cs

@@ -5,6 +5,7 @@
 using System.Collections.Immutable;
 using System.Linq;
 using System.Text.RegularExpressions;
+using System.Threading;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Editor.Shared.Extensions;
 using Microsoft.CodeAnalysis.Editor.UnitTests;
@@ -192,7 +193,7 @@ private protected static RunCodeActionParams CreateRunCodeActionParams(string co
 
             foreach (var document in workspace.Documents)
             {
-                var text = document.GetOpenTextContainer().CurrentText;
+                var text = solution.GetDocument(document.Id).GetTextSynchronously(CancellationToken.None);
                 foreach (var kvp in document.AnnotatedSpans)
                 {
                     locations.GetOrAdd(kvp.Key, CreateLocation)