Cannot run PHP unit tests for the new assignment module web services

Two problems are preventing PHP unit tests from working. These are:

1) The tests generator cannot create an instance of the assign module because the assign_add_instance constructor in mod/assign/lib.php does not allow a null mod_assign_mod_form parameter. This can be resolved with the following modification

function assign_add_instance(stdClass $data, mod_assign_mod_form $form = null)

2) mod/assign/locallib.php needs to be modified to resolve the problem described in MDL-27968. The proposed solution is to lazily initialise the renderer which could be done by creating a new private function get_output which performs the initialisation and then replacing all calls to $this->output with a call to get_output