tests: kernel/msgq_api: join threads after each test

On SMP systems, threads going through k_thread_abort() may still
be running while the test moves on to the next one. This creates
some interferences and may result in the next test failing. So
after each test, we need to do k_thread_join() on those threads
to make sure they are no longer active.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>