tests: provide test_debugfs_get/set_value() helpers

tests: provide test_debugfs_get/set_value() helpers

Since linux v5.1 gpio-mockup simulates pull-up/down resistors for dummy
lines. The current pull can be set using debugfs. Since we can no longer
verify if the character device interface properly sets line values, the
debugfs interface now also allows to read line values. This way we can
verify if the ABI works correctly using different code paths.

Provide functions for reading and setting values of mockup GPIO lines.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>