tests: arm_runtime_nmi: convert to ztest

- file does not use ztest asserts
- separated the main file into two files:
    - main.c, which has ztest entry
    - arm_runtime_nmi.c, which has the original routine

JIRA: ZEP-2382

Signed-off-by: Niranjhana N <niranjhana.n@intel.com>