tests/sys/netpfil/pf/sctp: make use of required.kmods

This greatly speeds up skipping a test that can't be run.

Sponsored by: ConnectWise
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D53215

(cherry picked from commit 7648d2ebda22fe21ed385cc7d76813a1a9c17c99)