Enable TestPreviewDiagnosticTaggerInPreviewPane.

We no longer have any artifacts for this.  Going to try to enable and see what
happens.

Fixes #14444.

src/EditorFeatures/Test/Preview/PreviewWorkspaceTests.cs

@@ -199,7 +199,7 @@ public async Task TestPreviewDiagnosticTagger()
             }
         }
 
-        [WpfFact(Skip = "https://github.com/dotnet/roslyn/issues/14444")]
+        [WpfFact]
         public async Task TestPreviewDiagnosticTaggerInPreviewPane()
         {
             using (var workspace = TestWorkspace.CreateCSharp("class { }", exportProvider: EditorServicesUtil.ExportProvider))