Add more mono trickery

tests/Avalonia.Markup.UnitTests/Parsers/ExpressionObserverBuilderTests_AttachedProperty.cs

@@ -117,9 +117,11 @@ namespace Avalonia.Markup.UnitTests.Parsers
             var result = run();
             result.Item1.Subscribe(x => { });
 
-            GC.Collect();
+            // Mono trickery
+            GC.Collect(2);
             GC.WaitForPendingFinalizers();
-            GC.Collect();
+            GC.WaitForPendingFinalizers();
+            GC.Collect(2);
 
             Assert.Null(result.Item2.Target);
         }