pf tests: test SCTP pfsync

Ensure that SCTP connections survive a failover to the backup pf
instance.

MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D40869

(cherry picked from commit b7a9a5773a0fdf9b9c7839d79d76dbc6b69a6c4b)