tests: kernel: interrupt: Rework nested interrupt test

The current nested interrupt test implementation is both buggy and
fundamentally flawed because it does not trigger a higher priority
interrupt from a lower priority interrupt context and relies on the
system timer interrupt, which is not fully governed by the test;
moreover, the current implementation does not properly validate the
test results and can report success if no interrupt is triggered and
serviced at all.

This commit reworks this test to have the following well-defined
and logical procedure:

1. [thread] Trigger IRQ 0 (lower priority)
2. [isr0] Set ISR 0 result token and trigger IRQ 1 (higher priority)
3. [isr1] Set ISR 1 result token and return
4. [isr0] Validate ISR 1 result token and return
5. [thread] Validate ISR 0 result token

The reworked test scenario ensures that the interrupt nesting works
properly and any abnormal conditions are detected (e.g. interrupts not
triggering at all, or ISR 1 not being nested under ISR 0).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>