tests/kernel/fatal: Don't swap while locked

This test takes an interrupt lock and tries to call z_swap_unlocked()
while holding it.  That's not legal (in the general case it means
you're breaking a caller's lock!), though in this particular case it
was safe because we'll never return to this.

Regardless, there is a natural z_swap_irqlock() that releases the lock
atomically.  Use that.

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