tests: drivers: comparator: add gpio_loopback test suite

Add test suite which uses GPIO loopback to produce a "very low" and
"very high" voltage at the positive input of the comparator using the
output of a GPIO. The negative input is set to a voltage between the
high and low output voltages of the GPIO using whichever internal
reference is available to the comparator.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>