tests: kernel: metairq: Ensure meta-IRQ can preempt a cooperative thread

The intention of the test is to have a meta-IRQ preempt a cooperative
thread, then ensure the same cooperative thread is resumed back to instead
of a higher priority thread. Thus, the test needs to assert that the lower
priority cooperative thread is not yet complete when it makes the higher
priority thread ready to run.

This change reveals bugs like #80574, where the meta-IRQ is unable to
preempt a cooperative thread.

Signed-off-by: Kalle Kietäväinen <kalle.kietavainen@silabs.com>