tests/mac_portacl: restore sysctls after modification

While here, use 'required_kmods' instead of ad-hoc
checks.

Signed-off-by: Siva Mahadevan <me@svmhdvn.name>

Reviewed by: markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Pull Request: https://github.com/freebsd/freebsd-src/pull/1855

(cherry picked from commit 170aac93479467334062813c1175a80200400b79)