[qutebrowser] xvfb/opengl error running tests

[Ryan Roden-Corrent]

Tests are failing on my machine due to the following warning:

    QtWarningMsg: QXcbIntegration: Cannot create platform OpenGL context,
    neither GLX nor EGL are enabled

This only happens on my desktop with an Nvidia card, my laptop with integrated
graphics is fine. Has anyone else seen this? Is there a way to make the tests
ignore a particular warning?

It works fine if I don't use xvfb, but that gets kind of annoying.

-Ryan