tests/kernel: verify RUNTIME_NMI at runtime

This patch introduces a test that verifies the behavior of
CONFIG_RUNTIME_NMI at runtime.

The test is meant to be only for ARM Cortex-M targets.

Change-Id: I805a88e67fe47d396ac9e29e1275e5452a4b8a36
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>