Use portability/GTest.h

Summary: Don't use gtest/gtest.h directly.

Reviewed By: yfeldblum

Differential Revision: D25659806

fbshipit-source-id: a8dd0da814465a787ab7904bdd0256f22b6e3ed8

folly/experimental/coro/GtestHelpers.h

@@ -16,10 +16,9 @@
 
 #pragma once
 
-#include <gtest/gtest.h>
-
 #include <folly/experimental/coro/BlockingWait.h>
 #include <folly/experimental/coro/Task.h>
+#include <folly/portability/GTest.h>
 
 /**
  * This is based on the GTEST_TEST_ macro from gtest-internal.h. It seems that