tests/kernel/fatal/exception: Remove legacy irq_lock() usage

The irq_lock() API is a legacy API not to be used for synchronization
by new code, and in any case is only being used in cargo-cult fashion
here.  These test cases all do synchronous exceptions, there's
literally nothing to synchronize against.

    (And in this case they're exposing a legacy wart.  On platforms where:

    1. SMP=y, which causes irq_lock() to be implemented as a somewhat
       complicated global lock

    2. No ARCH_EXCEPT() macro is defined, which causes the kernel to
       use a fallback that simply aborts the current thread.

    ...this test will then abort a thread holding the lock, which will
    cause it to be orphaned (if it weren't a legacy API, the kernel
    should probably attempt to clean it up in k_thread_abort(), but it
    is, and it doesn't), so the next attempt to lock it will hang.
    And it's even worse, because this test builds with SMP=y and
    MP_NUM_CPUS=1, so the hand will happen with interrupts masked on a
    system with only one CPU, and everything will lock up solid.)

Fixes #41877

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