selftests: gpio: add test cases for gpio-sim

Add a set of tests for the new gpio-sim module. This is a pure shell
test-suite and uses the helper programs available in the gpio selftests
directory. These test-cases only test the functionalities exposed by the
gpio-sim driver, not those handled by core gpiolib code.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>