Avoid scheduling an unnecessary dispose task for EmptyAsyncToken

Fixes #22272

src/Features/Core/Portable/Shared/TestHooks/IAsyncToken.cs

@@ -18,6 +18,8 @@ private EmptyAsyncToken()
 
         public void Dispose()
         {
+            // Empty by design requirement: operations which use IAsyncToken are free to optimize code sequences by
+            // eliding calls to EmptyAsyncToken.Dispose() with the understanding that it doesn't do anything.
         }
     }
 }

src/Features/Core/Portable/Shared/TestHooks/TaskExtensions.cs

@@ -21,7 +21,7 @@ public static Task CompletesAsyncOperation(this Task task, IAsyncToken asyncToke
 
         public static Task CompletesTrackingOperation(this Task task, IDisposable token)
         {
-            if (token == null)
+            if (token == null || token == EmptyAsyncToken.Instance)
             {
                 return task;
             }