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

rolls back 71fcb75d011828e052ad6ad83302dcff02bddee2
and patches it.

tulip-control/tulip-control,tulip-control/tulip-control,necozay/tulip-control,necozay/tulip-control,necozay/tulip-control,necozay/tulip-control,tulip-control/tulip-control,necozay/tulip-control,tulip-control/tulip-control