Following on from b8c16c995b, extend the family tree being tested by
place_above and place_below a little, ensuring that subsurfaces can't be
placed above or below surfaces which are related to them, but aren't
their immediate parent or sibling.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>