Comment 1 for bug 1604711

The helper class come from horizon, do you see the same error when running horizon tests or the tests for other dashboards?

The code in that class probably needs a better exception handler on the first subprocess call:

 if not subprocess.call('which xdpyinfo > /dev/null 2>&1', shell=True):
    SCREEN_SIZE = subprocess.check_output('xdpyinfo | grep dimensions',
                                          shell=True).split()[1].split('x')
else:
    SCREEN_SIZE = (None, None)
    LOGGER.info("X11 isn't installed. Should use xvfb to run tests.")

Missing exception handler aside (which is an horizon bug), it looks like xvfb is required.