MDL-68806 backup: Fix faulty question attempt step restore test

Previously, the test was passing by fluke because all question attempt
steps had the same timecreated date. However, the $originaliterator only
contained the first step of the question, not all 3 attempts that were
being backed up and restored. This means we were comparing the
timecreated of the first backed-up step with the timecreated on the
third restored step. As of MDL-68806, the first step's timecreated is
question_attempt_step::TIMECREATED_ON_FIRST_RENDER, which shows up this
error.

This change gets the question_usage_by_activity object back from the
attempt object which contains all the attempt steps, rather than using
the one created in the test, which doesn't.

backup/moodle2/tests/restore_stepslib_date_test.php

@@ -413,7 +413,7 @@ public function test_question_attempt_steps_date_restore(): void {
         $attemptobj->process_grade_submission($timenow);
 
         $questionattemptstepdates = [];
-        $originaliterator = $quba->get_attempt_iterator();
+        $originaliterator = $attemptobj->get_question_usage()->get_attempt_iterator();
         foreach ($originaliterator as $questionattempt) {
             $questionattemptstepdates[] = ['originaldate' => $questionattempt->get_last_action_time()];
         }