Add tests for TCP_MD5 getsockopt

Add tests for TCP_MD5 getsockopt for ipv6 and ipv4. These will only run
when the kernel module is loaded, but will be in place if further
regressions are found.

Reviewed by: rscheff
MFC after: 3 days
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35532