Minimize D-Bus requirements for tests (#5265)

This PR minimizes the D-Bus requirements for tests. It does this by
using dbus-daemon directly instead of dbus-launch. The latter is meant
for graphical applications and therefor has X11 dependencies. It also
leaves the D-Bus daemon running after the tests are done. This will
accumulate dbus-daemon processes over time which is not ideal.

I've also considered using dbus-run-session since it is meant to launch
processes with a private D-Bus session. For Python tests one could
launch it like so:
dbus-run-session -- python3 -m pytest ...

Then `DBUS_SESSION_BUS_ADDRESS` would be used automatically by the
`MessageBus` class. However, to keep the current behavior of the tests,
launching the D-Bus daemon manually is the better option.

.github/workflows/ci.yaml

@@ -313,7 +313,7 @@ jobs:
       - name: Install additional system dependencies
         run: |
           sudo apt-get update
-          sudo apt-get install -y --no-install-recommends libpulse0 libudev1 dbus dbus-x11
+          sudo apt-get install -y --no-install-recommends libpulse0 libudev1 dbus-daemon
       - name: Register Python problem matcher
         run: |
           echo "::add-matcher::.github/workflows/matchers/python.json"