tests: make screenshooting return a buffer

tests: make screenshooting return a buffer

Screenshooting does not involve creating a wl_surface, so using struct
surface is superfluous.

Return a struct buffer instead. It could have been just a
pixman_image_t, but setting up proper destruction would be a bit more
work. Should not hurt to keep the wl_buffer around until the user is
ready to free the image.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>