use 'Agg' backend in test to avoid X11 forwarding over ssh

rolls back 71fcb75d011828e052ad6ad83302dcff02bddee2
and patches it.
rm arg "show" from PPP.plot

following https://github.com/tulip-control/polytope:
	6f518cc5dfae4c710c688de35468f764835c6f4e