tests: gpiomon: more test cases
tests: gpiomon: more test cases

Add various test cases for gpiomon. Testing invalid input, active-low
mode, termination on signals etc.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod