core: sanitize the output values in gpiod_line_config_set_output_values()

core: sanitize the output values in gpiod_line_config_set_output_values()

We check the output values for invalid ones in
gpiod_line_settings_set_output_value() but in
gpiod_line_config_set_output_values() we just accept all indiscriminately.
Factor out the common checking code into a helper function, use it in both
functions and add a relevant test case.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>