MDL-60306 calendar: Fix rrule_manager unit test

* The calculation of the expected date should depend on the comparison
with the start date's date and not it's exact timestamp.

calendar/tests/rrule_manager_test.php

@@ -872,7 +872,7 @@ class core_calendar_rrule_manager_testcase extends advanced_testcase {
         $records = $DB->get_records('event', ['repeatid' => $this->event->id], 'timestart ASC', 'id, repeatid, timestart');
         $expecteddate = new DateTime('first Monday of this month');
         // Move to the next interval's first Monday if the calculated start date is after this month's first Monday.
-        if ($expecteddate->getTimestamp() < $startdatetime->getTimestamp()) {
+        if ($expecteddate->getTimestamp() < $startdate->getTimestamp()) {
             $expecteddate->add($interval);
             $expecteddate->modify('first Monday of this month');
         }