MDL-45606 Unit test fix for MySQL and added a comment about the removed code.

lib/phpunit/classes/util.php

@@ -106,6 +106,9 @@ class phpunit_util extends testing_util {
         // Stop any message redirection.
         phpunit_util::stop_phpmailer_redirection();
 
+        // We used to call gc_collect_cycles here to ensure desctructors were called between tests.
+        // This accounted for 25% of the total time running phpunit - so we removed it.
+
         // Show any unhandled debugging messages, the runbare() could already reset it.
         self::display_debugging_messages();
         self::reset_debugging();

question/engine/tests/questionusage_autosave_test.php

@@ -685,4 +685,10 @@ class question_usage_autosave_test extends qbehaviour_walkthrough_test_base {
 
         $this->delete_quba();
     }
+
+    protected function tearDown() {
+        // This test relies on the destructor for the second DB connection being called before running the next test.
+        // Without this change - there will be unit test failures on "some" DBs (MySQL).
+        gc_collect_cycles();
+    }
 }