Enable test.

src/VisualStudio/IntegrationTest/IntegrationTests/CSharp/CSharpReferenceHighlighting.cs

@@ -41,7 +41,7 @@ class {|definition:C|}
             VerifyNone("void");
         }
 
-        [Fact(Skip = "https://github.com/dotnet/roslyn/issues/19059"), Trait(Traits.Feature, Traits.Features.Classification)]
+        [Fact, Trait(Traits.Feature, Traits.Features.Classification)]
         public void WrittenReference()
         {
             var markup = @"

src/VisualStudio/IntegrationTest/IntegrationTests/VisualBasic/BasicReferenceHighlighting.cs

@@ -22,7 +22,7 @@ public BasicReferenceHighlighting(VisualStudioInstanceFactory instanceFactory)
         {
         }
 
-        [Fact(Skip = "https://github.com/dotnet/roslyn/issues/19059"), Trait(Traits.Feature, Traits.Features.Classification)]
+        [Fact, Trait(Traits.Feature, Traits.Features.Classification)]
         public void Highlighting()
         {
             var markup = @"