tests: arm: zero latency irqs: fix the derivation of free NVIC line

The commit fixes the way we determine an available
NVIC IRQ line to perform the zero-latency IRQ test.
The test can now run properly on SOCs that do not
have a continous set of implemented NVIC IRQ lines.

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