kernel/sched: Panic on aborting essential threads

Documentation specifies that aborting/terminating/exiting essential
threads is a system panic condition, but we didn't actually implement
that and allowed it as for other threads. At least one app wants to
exploit this documented behavior as a "watchdog" kind of condition,
and that seems reasonable.  Do what we say we're supposed to do.

This also includes a small fix to a test, which seemed like it was
written to exercise exactly this condition.  Except that it failed to
detect whether or not a system fatal error was actually signaled and
was (incorrectly) indicating "success".  Check that we actually enter
the handler.

Fixes #45545

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>