librt: add test (bug 28213)

This test implements following logic:
1) Create POSIX message queue.
   Register a notification with mq_notify (using NULL attributes).
   Then immediately unregister the notification with mq_notify.
   Helper thread in a vulnerable version of glibc
   should cause NULL pointer dereference after these steps.
2) Once again, register the same notification.
   Try to send a dummy message.
   Test is considered successfulif the dummy message
   is successfully received by the callback function.

Signed-off-by: Nikita Popov <npv1310@gmail.com>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>