tests: mem_protect: cleanup

This suite was fairly messy and very unstable on how it re-used
kernel objects.

* Unnecessary ztest_test_pass() or self-aborts removed
* k_thread_join() now used to wait for child thread completion,
  instead of a strange use of a semaphore which was effectively
  a 10ms sleep
* Barriers simplified
* the number of thread objects in kobject.c is now drastically reduced
* test case function names are now descriptive and made static if
  only used in local scope in kobject.c
* SMP no longer disabled

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