west: runners: Fix verify warning from nrfjprog

nrfjprog prints out a warning if a device is programed without the
--verify option, which can be confusing.
So added the option when programming with nrfjprog.
Then west flash can be used without getting a warning.
Also changed the tests to only accept programming with --verify
option.

Signed-off-by: Markus Swarowsky <markus.swarowsky@nordicsemi.no>