selftests: media_tests: Add new subtest to video_device_test

Add new subtest to video_device_test to cover the VIDIOC_G_PRIORITY
and VIDIOC_S_PRIORITY ioctl calls. This test tries to set the priority
associated with the file descriptior via ioctl VIDIOC_S_PRIORITY
command from V4L2 API. After that, the test tries to get the new
priority via VIDIOC_G_PRIORITY ioctl command and compares the result
with the v4l2_priority it set before. At the end, the test restores the
old priority.

This test will increase the code coverage for video_device_test, so
I think it might be useful. Additionally, this patch will refactor the
video_device_test a little bit, according to the new functionality.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>