Since we use pytest-xvfb, Xvfb is automatically started and stopped by
pytest. The herbstluftwm also runs in the "outer" Xvfb and is not
actually available inside the tests, as the tests run in another
display, while herbstluftwm runs in :0.
This patch removes herbstluftwm completely (it was not used until now,
so there shouldn't be a problem), and doesn't start Xvfb (the package is
still needed for pytest-xvfb to work).
Additionally, it adds the "xauth" package on debian, because we'll need
it anyway sooner or later. It's automatically included in Ubuntu and
Arch because xvfb depends on it, but on debian, xauth is only
recommended and not installed automatically.
Upstream Arch now updated to Qt 5.6 (which means it works with Xvfb), and using
debug packages means the image is a lot bigger and builds slower, so let's use
the normal packages from [extra].
