tests: arch: arm_irq_vector_table: enable test for nRF51x platforms

This commit enables running the test in CI for nRF51-based
platforms, which require special IRQ lines for the IRQs
used in the test. An additional customization is needed
for QEMU_CORTEX_M0, which uses a different system timer
based on TIMER0 peripheral.

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