Update buck build in Visual Studio

Summary:
- Always launch buck build for Build, but copy native library when it was changed
- Rename Facebook.Yoga.Mac.Test to Facebook.Yoga.Mac.Sample
- Add Facebook.Yoga.Mac.Tests for NUnit test.
  - "Run Unit Tests" menu item was activated for the target, but Mono still can't find libyoga.dylib
Closes https://github.com/facebook/yoga/pull/332

Reviewed By: emilsjolander

Differential Revision: D4408799

Pulled By: splhack

fbshipit-source-id: b3f5f9ebd8265cc152ca859176afbf54efa8f170

csharp/Mac/Facebook.Yoga.Mac.Tests/Main.cs

@@ -0,0 +1,20 @@
+/**
+ * Copyright 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the license found in the
+ * LICENSE-examples file in the root directory of this source tree.
+ */
+
+using System;
+using System.Reflection;
+
+namespace Facebook.Yoga.Mac.Tests
+{
+    static class MainClass
+    {
+        static void Main(string[] args)
+        {
+        }
+    }
+}