resolv_test: Fix racy exit check, remove mutexes, and reduce output

Instead of polling nleft[i] (without appropriate memory barriers!) and
using sleep() to detect the exit just call pthread_join() on all threads.

Also replace the use of a mutex that guarding the increments with atomic
fetch_add. This should reduce the runtime of this test on SMP systems.

Finally, remove all the debug printfs unless DEBUG_OUTPUT is set in
the environment.

Test Plan: still fails sometimes on qemu (but maybe less often?)
Reviewed By: jhb
Differential Revision: https://reviews.freebsd.org/D29390