Reorgnaize C++ tests

Summary:
This does some preprataion for the Yoga CMake Build. The main change is removing the dedicated testutil top-level-directory and static library. This contains a method to count nodes using the event functions exposed to C++, along with a Java binding for the test utility (since the events don't have a Java binding). It is only used in a single place in a way that isn't very useful, so it simplifies things to treat is as source in the existing C++ test library.

This also separates the hand-written and generated UTs, like we are doing in the JS directory in D42207782.

Reviewed By: christophpurrer

Differential Revision: D42247762

fbshipit-source-id: f8a270e99d0315ba7fc608f2471333e7a7be9d79

java/tests/com/facebook/yoga/YogaNodeTest.java

@@ -32,9 +32,7 @@ public class YogaNodeTest {
 
   @Test
   public void testInit() {
-    TestUtil.startCountingNodes();
     final YogaNode node = createNode();
-    assertEquals(1, TestUtil.stopCountingNodes());
   }
 
   @Test