Input: tests - add test to cover all input_grab_device() function

Currently input_grab_device() isn't covered by any tests
Thus, adding a test to cover the cases:
1. The device is grabbed successfully
2. Trying to grab a device that is already grabbed by another input
   handle

Signed-off-by: Dana Elfassy <dangel101@gmail.com>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20230522215514.722564-1-dangel101@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>