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/dbus-appinfo.c

@@ -280,11 +280,7 @@ main (int argc, char **argv)
 {
   g_test_init (&argc, &argv, NULL);
 
-  session_bus_up ();
-
   g_test_add_func ("/appinfo/dbusappinfo", test_dbus_appinfo);
 
-  session_bus_down ();
-
-  return g_test_run ();
+  return session_bus_run ();
 }