ztest: cleanup phase should reset the timeout queue.

Reusing the k_thread structure requires a cleanup of all essential
info. We need to remove the ztest_thread information from the timeout_q.

If left untouched, when a tick occurs the thread's delta_ticks_from_prev
would be corrupted. This inturn causes a chain reaction of problems.
Thus once the unit test is completed the timeout_q is scrubbed.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>