tests: Use dbus-run-session instead of dbus-launch

dbus-launch is X11-specific and contains a lot of legacy code to
support X11 autolaunching. It should not be part of the Wayland
future.

Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=836067

https://bugzilla.gnome.org/show_bug.cgi?id=771455