'Run a command, and return the output string for scrutiny'

def _help_output(self, args):

    output = six.StringIO()
    with mock.patch('certbot.main.sys.stdout', new=output):
        with mock.patch('certbot.main.sys.stderr'):
            self.assertRaises(SystemExit, self.parse, args, output)
    return output.getvalue()
