Fix race condition in Mutex LockableThreaded test

If the thread starts slowly enough it will succeed in taking the lock,
leading to a deadlock. We saw this test hanging on some of our bots
inside of pthread's lock acquire.

Author: benlangmuir

unittests/runtime/Mutex.cpp

@@ -153,9 +153,8 @@ TEST(MutexTest, LockableThreaded) {
 
   mutex.lock();
   auto thread = std::thread([&] { ASSERT_FALSE(mutex.try_lock()); });
-  std::this_thread::sleep_for(std::chrono::milliseconds(10));
-  mutex.unlock();
   thread.join();
+  mutex.unlock();
 
   thread = std::thread([&] {
     ASSERT_TRUE(mutex.try_lock());