C++ 17 style nested namespaces (#1326)

Summary:
X-link: https://github.com/facebook/react-native/pull/38304

Pull Request resolved: https://github.com/facebook/yoga/pull/1326

For better readability

Reviewed By: christophpurrer

Differential Revision: D47384926

fbshipit-source-id: 2f60d50a185331b3624d45d1fc45f98d504b3034

tests/util/TestUtil.h

@@ -11,9 +11,7 @@
 
 #include <functional>
 
-namespace facebook {
-namespace yoga {
-namespace test {
+namespace facebook::yoga::test {
 
 struct YOGA_EXPORT TestUtil {
   static void startCountingNodes();
@@ -26,6 +24,4 @@ struct ScopedEventSubscription {
   ~ScopedEventSubscription();
 };
 
-} // namespace test
-} // namespace yoga
-} // namespace facebook
+} // namespace facebook::yoga::test