tests: Initialize dbus before gtk

tests: Initialize dbus before gtk

These days initilizing gtk may create a connection to the sesson bus,
so we have to initialize GTestDBus before initalizing gtk, or we'll
use the address of the "real" session bus (and remember that in the
global).

To further muck things up, g_test_dbus_up() resets important env
vars like DISPLAY and XDG_RUNTIME_DIR, which we have to re-set.