selftests: gpio: rework and simplify test implementation

The GPIO mockup selftests are overly complicated with separate
implementations of the tests for sysfs and cdev uAPI, and with the cdev
implementation being dependent on tools/gpio and libmount.

Rework the test implementation to provide a common test suite with a
simplified pluggable uAPI interface.  The cdev implementation utilises
the GPIO uAPI directly to remove the dependence on tools/gpio.
The simplified uAPI interface removes the need for any file system mount
checks in C, and so removes the dependence on libmount.

The rework also fixes the sysfs test implementation which has been broken
since the device created in the multiple gpiochip case was split into
separate devices.

Fixes: 8a39f597

 ("gpio: mockup: rework device probing")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>