test: fix failing tests

Broken since 69d8ea4a because our fake screen
didn't have a root window and writing the XKB rules prop would happily
segfault. Fix this by setting up the required bits.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit fde27b9b)