tests: drivers: regulator: add voltage levels test

Add a new test that checks if the configured regulator voltage matches
the _real_ voltage obtained using an ADC. The test is generic so that
future regulators can be tested easily by just adding a new board
overlay file and a new test case entry.

Tested on nPM6001 using:

./scripts/twister \
	--device-testing \
	--device-serial /dev/ttyACM0 \
	-p nrf52840dk_nrf52840 \
	-X regulator_output_to_adc \
	-T tests/drivers/regulator/voltage

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>