tests: move dbus session bus handling into runtests.py
This allows us to remove code duplication in autotools/distutils.
This also fixes the noisy dbus output during tests because dbus-run-session forwarded dbus logging output to stderr.
This allows us to remove code duplication in autotools/distutils.
This also fixes the noisy dbus output during tests because dbus-run-session forwarded dbus logging output to stderr.