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/gdbus-proxy-threads.c

@@ -205,11 +205,6 @@ test_proxy (void)
 
   session_bus_up ();
 
-  /* TODO: wait a bit for the bus to come up.. ideally session_bus_up() won't return
-   * until one can connect to the bus but that's not how things work right now
-   */
-  usleep (500 * 1000);
-
   loop = g_main_loop_new (NULL, TRUE);
 
   connection = g_bus_get_sync (G_BUS_TYPE_SESSION,
@@ -240,12 +235,11 @@ int
 main (int   argc,
       char *argv[])
 {
-  g_unsetenv ("DISPLAY");
-  g_setenv ("DBUS_SESSION_BUS_ADDRESS", session_bus_get_temporary_address (), TRUE);
-
   g_type_init ();
   g_test_init (&argc, &argv, NULL);
 
+  g_test_dbus_unset ();
+
   g_test_add_func ("/gdbus/proxy/vs-threads", test_proxy);
 
   return g_test_run();