pf: Pass v6 packets to the divert socket

There is no particular limitation of divert sockets with respect to
IPv6, and the pf.conf man page does not mention the restriction to IPv4.

Extend the divert-to regression tests to exercise the v6 case.

Reviewed by: igoro, kp, glebius
MFC after: 3 weeks
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54847