tests: add test_seat_release() for symmetry

Add test_seat_release() as the counterpart of test_seat_init() instead
of open-coding it. This helps adding more code to test_seat_release()
later.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>