gpio: add kunit test cases for the GPIO subsystem

gpio: add kunit test cases for the GPIO subsystem

Add a module containing kunit test cases for GPIO core. The idea is to
use it to test functionalities that can't easily be tested from
user-space with kernel selftests or GPIO character device test suites
provided by the libgpiod package.

For now add test cases that verify software node based lookup and ensure
that a GPIO provider unbinding with active consumers does not cause a
crash.

Reviewed-by: David Gow <david@davidgow.net>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260522-gpiolib-kunit-v3-3-b15fe6987430@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>