Input: Add KUnit tests for some of the input core helper functions

The input subsystem doesn't currently have any unit tests, let's add a
CONFIG_INPUT_KUNIT_TEST option that builds a test suite to be executed
with the KUnit test infrastructure.

For now, only three tests were added for some of the input core helper
functions that are trivial to test:

* input_test_polling: set/get poll interval and set-up a poll handler.

* input_test_timestamp: set/get input event timestamps.

* input_test_match_device_id: match a device by bus, vendor, product,
version and events capable of handling.

But having the minimal KUnit support allows to add more tests and suites
as follow-up changes. The tests can be run with the following command:

$ ./tools/testing/kunit/kunit.py run --kunitconfig=drivers/input/tests/

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Enric Balletbo i Serra <eballetbo@redhat.com>
config: powerpc-allnoconfig (https://download.01.org/0day-ci/archive/20230330/202303301815.kRKFM3NH-lkp@intel.com/config)
Link: https://lore.kernel.org/r/20230330081831.2291351-1-javierm@redhat.com

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>