Simplify writing tests, (mostly) use private session bus and fix leaks found by that

I needed a way to not have test mess with the session bus for !780 (closed) so i switched most test to g_test_dbus() and fixed the resource leaks found by that. On top of that i simplified writing new tests. Split out of !780 (closed) to not make that grow out of control (the tests there also need newer glib due to https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2120 .

Signed-off-by: Guido Günther guido.gunther@puri.sm