Avoid XVFB collisions
Avoid XVFB collisions

Add parameter to xvfb-run to "try to get a free server". This is
necessary to avoid collisions when running the tests in batch that lead
to spurious failures.

patmarion/director,manuelli/director,patmarion/director,openhumanoids/director,patmarion/director,manuelli/director,openhumanoids/director,openhumanoids/director,openhumanoids/director,mitdrc/director,manuelli/director,patmarion/director,mitdrc/director,openhumanoids/director,manuelli/director,patmarion/director,mitdrc/director,manuelli/director,mitdrc/director,mitdrc/director