Use GTestDBus in all GDBus unit tests

To make port easier, this rewrites dbus-sessionbus.c using a
GTestDBus singleton internally.

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

gio/tests/gdbus-auth.c

@@ -269,11 +269,6 @@ main (int   argc,
   g_type_init ();
   g_test_init (&argc, &argv, NULL);
 
-  /* all the tests use a session bus with a well-known address that we can bring up and down
-   * using session_bus_up() and session_bus_down().
-   */
-  g_unsetenv ("DISPLAY");
-  g_setenv ("DBUS_SESSION_BUS_ADDRESS", session_bus_get_temporary_address (), TRUE);
   session_bus_up ();
 
   g_test_add_func ("/gdbus/auth/client/EXTERNAL",         auth_client_external);