improve/debloat mutex unlock error checking in pthread_cond_wait
improve/debloat mutex unlock error checking in pthread_cond_wait

we're not required to check this except for error-checking mutexes,
but it doesn't hurt. the new test is actually simpler/lighter, and it
also eliminates the need to later check that pthread_mutex_unlock
succeeds.

chardi/musl,davidlazar/musl,chardi/musl,davidlazar/musl,chardi/musl