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-peer.c

@@ -1804,8 +1804,6 @@ main (int   argc,
   g_type_init ();
   g_test_init (&argc, &argv, NULL);
 
-  g_unsetenv ("DBUS_SESSION_BUS_ADDRESS");
-
   introspection_data = g_dbus_node_info_new_for_xml (test_interface_introspection_xml, NULL);
   g_assert (introspection_data != NULL);
   test_interface_introspection_data = introspection_data->interfaces[0];