tests/kernel/fatal: Work around historical API misuse

This test was written to do a k_oops() in the main thread.  That's an
essential thread, and aborting it is actually a system panic now.  The
test was written contra the docs on this, but it worked fine for
years.

Just reflag the thread as a simple workaround rather than trying
anything fancy. This is a very simple test.

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