tests/kernel/fatal: Fix wait-for-interrupt delay in stack check test

Contrary to the comment in code, this test is NOT, in fact, compiled
with a traditional ticked kernel.  Spinning won't work reliably
because interrupts won't necessarily be delivered when you expect.
This test case would fail spuriously as I moved things around when
debugging.

Doing it right (using a k_timer in this case) is actually less code
anyway.

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