test(display): Give the live tests after the Openbox one their DISPLAY back
live_openbox_is_read_and_restarted points DISPLAY at a private Xvfb it
stops at the end, and left it there, so x11::tests' live_output_layout
and live_resize_round_trip, run after it in the same process, opened a
dead display and failed. It restores the DISPLAY the process had. All
seven live tests pass in one run on a slot's Xvfb.