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

yoga/events.cpp

@@ -24,6 +24,10 @@ std::function<Event::Subscriber>& globalEventSubscriber() {
 
 } // namespace
 
+void Event::reset() {
+  globalEventSubscriber() = nullptr;
+}
+
 void Event::subscribe(std::function<Subscriber>&& subscriber) {
   if (globalEventSubscriber() != nullptr) {
     throw std::logic_error(

yoga/events.h

@@ -35,6 +35,8 @@ struct Event {
     };
   };
 
+  static void reset();
+
   static void subscribe(std::function<Subscriber>&& subscriber);
 
   template <Type E>