scripts: runners: tests: nrf: Minor tweaks

- Rename the test file to test_nrf.py in preparation for future tests
  covering nrfutil as well
- Rename the test folder from nrfjprog/ to nrf/
- Change the id_fn returned string formatting so that it's compatible
  with executing a single test case via -k as a parameter (no spaces or
  commas)

You can now execute:
python -m pytest tests/test_nrf.py -k \
'test_nrfjprog_init[NRF51-pin_reset-override_snr-sector[anduicr]\
_erase-no_recover-no_tool_opt]'

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>