Test suite's display_in_use is not compatible with rootless X.Org
Originally created by @intrigeri on #10460 (Redmine)
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 start_xvfb
runs sleep 1
in an endless loop. I haven’t found
another way to identify whether a display is in use yet.