Add YogaConfig unit tests

Summary:
- depends on #497
Closes https://github.com/facebook/yoga/pull/498

Reviewed By: emilsjolander

Differential Revision: D4796199

Pulled By: splhack

fbshipit-source-id: c63f23da11a719b36c0d49e954b29c0016cad8c7

csharp/tests/Facebook.Yoga/YogaNodeTest.cs

@@ -301,7 +301,7 @@ namespace Facebook.Yoga
             Assert.AreEqual(90.Pt(), node4.MaxHeight);
         }
 
-        private void ForceGC()
+        public static void ForceGC()
         {
             GC.Collect(GC.MaxGeneration);
             GC.WaitForPendingFinalizers();