tests: kernel: interrupt: make test work with any available NVIC IRQ

This commit re-works the test for the ARM architecture,
so that it can work with any available NVIC IRQ, not
bound to use the last 2 NVIC lines. It makes use of
the dynamic IRQ feature.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>