Delete associated testcases when the question is deleted.

questiontype.php

@@ -304,6 +304,9 @@ class qtype_stack extends question_type {
 
     public function delete_question($questionid, $contextid) {
         global $DB;
+        $DB->delete_records('qtype_stack_qtest_expected', array('questionid' => $questionid));
+        $DB->delete_records('qtype_stack_qtest_inputs',   array('questionid' => $questionid));
+        $DB->delete_records('qtype_stack_qtests',         array('questionid' => $questionid));
         $DB->delete_records('qtype_stack_prt_nodes',      array('questionid' => $questionid));
         $DB->delete_records('qtype_stack_prts',           array('questionid' => $questionid));
         $DB->delete_records('qtype_stack_inputs',         array('questionid' => $questionid));