[libc++] Mark two std::timed_mutex tests as flaky

Author: ldionne

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.timedmutex.requirements/thread.timedmutex.class/try_lock_for.pass.cpp

@@ -8,6 +8,8 @@
 //
 // UNSUPPORTED: libcpp-has-no-threads
 
+// ALLOW_RETRIES: 2
+
 // <mutex>
 
 // class timed_mutex;

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.timedmutex.requirements/thread.timedmutex.class/try_lock_until.pass.cpp

@@ -8,6 +8,8 @@
 //
 // UNSUPPORTED: libcpp-has-no-threads
 
+// ALLOW_RETRIES: 2
+
 // <mutex>
 
 // class timed_mutex;