MDL-55611 calendar: Fix unit test filter

The event test factory uses a closure to return only every other record.
This was previously working based on ID but MSSQL starts from an even
number rather than an odd number.

This change changes that to use a static variable which keeps a count of
the records instead and only returns every other record in this fashion,
removing the dependance upon ID.

calendar/tests/event_vault_test.php

@@ -208,7 +208,9 @@ class core_calendar_event_vault_testcase extends advanced_testcase {
         $user = $this->getDataGenerator()->create_user();
         // The factory will skip events with even ids.
         $factory = new action_event_test_factory(function($actionevent) {
-            return ($actionevent->get_id() % 2) ? false : true;
+            static $count = 0;
+            $count++;
+            return ($count % 2) ? true : false;
         });
         $strategy = new raw_event_retrieval_strategy();
         $vault = new event_vault($factory, $strategy);