tests: clean up fatal error handlers

- k_sys_fatal_error_handler() can return on all platforms,
  indicating that the faulting thread should be aborted.
- Hang the system for unexpected faults instead of trying
  to keep going, we have no idea whether the system is even
  runnable.

Prevents infinite crash loops during tests.

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