Keep test excluded on desktop

Originally I thought this function could be ported to be portable. Under
the hood it relies on `SoapHexBinary` which is desktop only. Hence this
does need to be a desktop only test.

src/Compilers/CSharp/Test/Emit/Attributes/InternalsVisibleToAndStrongNameTests.cs

@@ -2231,9 +2231,7 @@ internal void Goo()
             }
         }
 
-// !!!DO NOT MERGE
-// TODO: FIX THIS 
-#if NET46
+#if !NETCOREAPP2_1
         [ConditionalFact(typeof(WindowsDesktopOnly), Reason = "https://github.com/dotnet/roslyn/issues/30152")]
         [WorkItem(399, "https://github.com/dotnet/roslyn/issues/399")]
         public void Bug399()