Test suite's display_in_use is not compatible with rootless X.Org
On current sid (at least when using GDM, not sure if the other DM's have caught up yet) X runs as non-root, and our usual ways to detect if
:0 is used don't work, so we try to start Xvfb on that display and it fails and then
sleep 1 in an endless loop. I haven't found another way to identify whether a display is in use yet.