sysctl: Add a regression test which runs sysctl -a

Run sysctl -a during the test suite so that KASAN/KMSAN have a chance to
catch something.

Inspired by https://jprx.io/cve-2024-54507/

Reviewed by: jhb, emaste
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D48659