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/buck-build.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+if buck --version >/dev/null 2>&1; then true; else
+  echo "SKIP: Need to install buck https://buckbuild.com/setup/getting_started.html"
+  exit 0
+fi
+buck build $1