Add test utilities for C++ and Java

Summary:
@public

Test utility on top of the new event system that maintains a counter of instantiated nodes. Meant to replace the global node counter.

Reviewed By: SidharthGuglani

Differential Revision: D15174855

fbshipit-source-id: 6998472f95a09b8da652257a26596164bdcf43d6

testutil/TestUtil.java

@@ -0,0 +1,19 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+package com.facebook.yoga;
+
+import com.facebook.soloader.SoLoader;
+
+class TestUtil {
+  static {
+    SoLoader.loadLibrary("yoga_testutil_jni");
+  }
+
+  static native void startCountingNodes();
+  static native int nodeCount();
+  static native int stopCountingNodes();
+}