tests/internal-screenshot: Fix test so it doesn't expect shell surfaces

We no longer have a race with shell startup because we create our own
colored surface and check that it's properly drawn.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
3 files changed