Add Workaround #if to GC test

Summary: Closes https://github.com/facebook/yoga/pull/474

Reviewed By: emilsjolander

Differential Revision: D4687978

Pulled By: splhack

fbshipit-source-id: 5b411e94f76c9846eadd2f01f0c8fd511fd6cdbf

csharp/tests/Facebook.Yoga/YogaNodeTest.cs

@@ -365,9 +365,8 @@ namespace Facebook.Yoga
             child = null;
         }
 
-#if !__UNIFIED__
+#if YOGA_ENABLE_GC_TEST
         [Test]
-#endif
         public void TestParentDestructor()
         {
             ForceGC();
@@ -388,6 +387,7 @@ namespace Facebook.Yoga
             Assert.AreEqual(instanceCount + 1, YogaNode.GetInstanceCount());
             parent.Insert(0, child);
         }
+#endif
 
         [Test]
         public void TestClearWithChildDestructor()