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

As of commit 85b0f9de11c3 ("capsicum: propagate rights on accept(2)")
a capability is generated from accept(cap_fd,...). Enable the
corresponding test code.

PR: 201052
Reviewed by: oshogbo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46994

(cherry picked from commit 6684779b321590c71f162390bcf28feee2a3b967)