Unit test `dirsizes_totara_test` fails in Totara

[agwells]

tests/check/dirsizes_totara_test.php causes a unit test failure in Totara, because it tests functionality that relies on Moodle 3.9 APIs. It should have a check for those that conditionally skips the test, like tests/check/tasklatencycheck_test.php has:

        if (!class_exists('core\check\check')) {
            $this->markTestSkipped();
        }