Tests: add session_bus_run() and use it where possible

This is to avoid having again the subtil bug in dbus-appinfo.c:
session_bus_down() was called before g_test_run() so the test was
running on the user's dbus session.

https://bugzilla.gnome.org/show_bug.cgi?id=697348

gio/tests/gdbus-export.c

@@ -1732,15 +1732,11 @@ int
 main (int   argc,
       char *argv[])
 {
-  gint ret;
-
   g_test_init (&argc, &argv, NULL);
 
   /* all the tests rely on a shared main loop */
   loop = g_main_loop_new (NULL, FALSE);
 
-  session_bus_up ();
-
   g_test_add_func ("/gdbus/object-registration", test_object_registration);
   g_test_add_func ("/gdbus/registered-interfaces", test_registered_interfaces);
   g_test_add_func ("/gdbus/async-properties", test_async_properties);
@@ -1748,10 +1744,5 @@ main (int   argc,
   /* TODO: check that we spit out correct introspection data */
   /* TODO: check that registering a whole subtree works */
 
-  ret = g_test_run();
-
-  /* tear down bus */
-  session_bus_down ();
-
-  return ret;
+  return session_bus_run ();
 }