pf: enable the syncookie feature for IPv6

When syncookie support was added to pf the relevant work was only done
in pf_test(), not pf_test6(). Do this now.

MFC after: 1 week
Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D41502

(cherry picked from commit 92d4152295bc670b80352b5fbe102565de99a4eb)