ztest: end tests more robustly

If a ztest test case creates child thread(s), and one of the
descendent threads invokes ztest_test_pass(), ztest_test_fail(),
or ztest_thread_skip(), only that descendent thread will be
aborted.

Then ztest will try to run the next scenario on the ztest_thread
which is already in use. This was causing corruption issues on
SMP systems, and possibly other subtle, hard-to-debug
situations.

This patch ensures that ztest_thread is always dead before
re-using it, as run_test() now attempts to join on it instead
of using a semaphore.

The ztest_test_* functions now ensure that the ztest_thread
is always aborted, in addition to the current thread.

This isn't perfect. If the testcase spawned other threads,
they will keep running. The most robust way to fix this is to
iterate over all non-essential threads in the system and abort
them. Unfortunately, Zephyr doesn't have a facility to do
this safely.

It would also be simpler to re-use thread objects if
k_thread_create() could detect whether the thread was already
active and abort it, but this is currently not possible
since k_thread_create() can be used with uninitialzed
thread object memory and no checks are possible. This
may be improved in the future, see #23030.

Fixes: #22738
Partial fix for: #24713

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>