[Bug libstdc++/57641] std::timed_mutex.try_lock_until() is broken

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57641

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's undefined behaviour because you try to lock the same mutex twice in the
same thread, see my testcase for a well-defined way to do it, calling
try_lock_until in a new thread.