scripts: runners: tests: nrf: Refactor and add nrfutil runner tests

Refactor the existing nrfjprog tests to make them compatible with the
new nrfutil runner implementation, and extend them so that they are
exectued for both tools.

There is missing verification of the JSON generated by the nrfutil
runner, whcih will be added in a follow-up commit.

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