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.
Otherwise the tests will fail with:
INVALID: process 975: D-Bus library appears to be incorrectly set up; failed to read machine uuid: UUID file '/etc/machine-id' should contain a hex string of length 32, not length 0, with no other text
INVALID: See the manual page for dbus-uuidgen to correct this issue.
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].
