FIX: Tests pass under windows

Finally got a decent Windows rig to test on. This gets tests passing, at least for the backends I have on my Windows Python3.4 install (`pyqt4`, `pyglet`, `glfw`).

Ready for review/merge.
@lcampagn if you're okay with this hack/workaround, feel free to merge -- having this in `master` will help me deal with #555.