Run unit tests before building Unity package

Summary:
- Enable destructor unit tests in Unity Editor
- Exit pack.sh immediately if a command exits with a non-zero status
- Run unit tests before building Unity package

Reviewed By: emilsjolander

Differential Revision: D4365773

fbshipit-source-id: 20d0a51f4cef791154c17d329ac36598dc333a60

csharp/tests/Facebook.Yoga/YogaNodeTest.cs

@@ -228,7 +228,6 @@ namespace Facebook.Yoga
             Assert.AreEqual(100, node0.MaxHeight);
         }
 
-#if !UNITY_EDITOR
         private void ForceGC()
         {
             GC.Collect(GC.MaxGeneration);
@@ -356,6 +355,5 @@ namespace Facebook.Yoga
                 return MeasureOutput.Make(120, 130);
             });
         }
-#endif
     }
 }