Work around F26 base image bug: g-i-s failing to run
    
    With the latest F26 base images, it seems like g-i-s fails to
    run at first login. This is clearly some kind of bug somewhere
    and I'll investigate it, but it shouldn't be causing the update
    tests to fail - we can still validly run the tests with g-i-s
    not running. So for now, adjust the _graphical_wait_login test
    to tolerate this behaviour when running update tests.