Docs: Add the namespace using.

Author: ping-yee

tests/system/Cookie/CookieTest.php

@@ -15,6 +15,7 @@
 use CodeIgniter\Test\CIUnitTestCase;
 use Config\Cookie as CookieConfig;
 use DateTimeImmutable;
+use DateTimeInterface;
 use DateTimeZone;
 use Generator;
 use LogicException;