capsicum-test: rights are propagated on sctp_peeloff(2)

As of commit 91a9e4e01dab ("capsicum: propagate rights on sctp_peeloff")
a capability is generated from sctp_peeloff(cap_fd,...). Enable the
corresponding test code.

PR: 201052
Reviewed by: oshogbo, tuexen
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47000