capsicum-test: skip SCTP tests if SCTP not available

Skip the test rather than failing if SCTP is not available (e.g., if
sctp.ko is not loaded).

Also submitted upstream as
https://github.com/google/capsicum-test/pull/61

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46999

(cherry picked from commit 1d83090d850f627b6d6ebe7837e807df5d29799f)