Fix thread leak in //folly/portability/test:pthread_test.

Summary: The test forgot to join the pthread.

Reviewed By: yfeldblum

Differential Revision: D22743458

fbshipit-source-id: 02231ce1e915b9f8679378eaf2ec18dc5dd67280

folly/portability/test/PThreadTest.cpp

@@ -59,6 +59,8 @@ TEST(PThreadTest, pthread_equal) {
   pthread_t thread;
   EXPECT_EQ(pthread_create(&thread, nullptr, mainFunc, nullptr), 0);
   EXPECT_EQ(pthread_equal(thread, self), 0);
+  void* exitCode = nullptr;
+  pthread_join(thread, &exitCode);
 }
 
 TEST(PThreadTest, pthread_self_on_pthread_thread) {