tests/kernel/threads: Correct essential thread abort test

Thread abort of essential threads had a buglet where the panic would
be signaled but, if caught by k_sys_fatal_error_handler(), the thread
would not actually be aborted.  This test wasn't validating that the
thread was actually dead (in fac, it RELIED on the thread to signal a
semaphore after its "abort"!).

Make sure it's dead and that the panic is caught.  Also add a case for
aborting _current which is a separate code path that needs coverage.

Signed-off-by: Andy Ross <andyross@google.com>