Add decorator for FreeBSD failing test due to pr18061

The test code locks mutexes from one thread and unlocks them from a
different one, which is not valid (it's either undefined behaviour or is
required to return an error).

See http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@195707 91177308-0d34-0410-b5e6-96231b3b80d8