[ticket/12350] Instantiated mock phpbb_dispatcher so that test doesn't return non-object error

In modules_test.php::test_modules_auth() , ```p_master::module_auth()``` calls the static ```module_auth``` function, but module_auth references phpbb_dispatcher, which may not have been instantiated. This causes ```Call to a member function trigger_event() on a non-object``` as mentioned in #1943

Ran into this while running HHVM's test runner (which can run in parallel). Let me know if a more formal write-up is needed.