[libc++] Mark the shared_future.wait_for test as being flaky

It is timing sensitive and it fails from time to time. If marking it as
flaky doesn't help, we can try tweaking the time outs.

Author: ldionne

libcxx/test/std/thread/futures/futures.shared_future/wait_for.pass.cpp

@@ -9,6 +9,8 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03
 
+// FLAKY_TEST.
+
 // <future>
 
 // class shared_future<R>