Add GTestDBus object

This is a helper to write unit tests using a private dbus-daemon.

session_bus_up/down() are now just wrappers around a GTestDBus singleton.

https://bugzilla.gnome.org/show_bug.cgi?id=672985

gio/tests/gmenumodel.c

@@ -1026,9 +1026,6 @@ main (int argc, char **argv)
   g_test_init (&argc, &argv, NULL);
   g_type_init ();
 
-  g_unsetenv ("DISPLAY");
-  g_setenv ("DBUS_SESSION_BUS_ADDRESS", session_bus_get_temporary_address (), TRUE);
-
   session_bus_up ();
 
   g_test_add_func ("/gmenu/equality", test_equality);