tests: give name for the test-shell background

Give a role and a label for the test desktop shell background surface.
This makes it easier reading scenegraph dumps and other surface related
debug messages in tests when you don't have to guess what this
mysterious "PID 0, surface ID 0" surface is.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>