[libc++] Allow retries on some slightly flaky mutex tests

Author: ldionne

libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_try_to_lock.pass.cpp

@@ -5,8 +5,9 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
-//
+
 // UNSUPPORTED: libcpp-has-no-threads
+// ALLOW_RETRIES: 2
 
 // <mutex>
 

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.mutex.requirements.mutex/thread.mutex.recursive/lock.pass.cpp

@@ -5,8 +5,9 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
-//
+
 // UNSUPPORTED: libcpp-has-no-threads
+// ALLOW_RETRIES: 2
 
 // <mutex>