add mutex_unlock_and_sleep()
add test for mutex_unlock_and_sleep()
Merge pull request #728 from BytesGalore/add_unlock_mutex_and_sleep

core:mutex: enable atomic execution of unlock a mutex and sleep for a thread
Add `pthread_barrier_*` functions

Compare [`pthread_barrier_init`][1].

  [1]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_barrier_init.html
Add pthread_barrier test