pf tests: Verify (tcp) checksum modification on unaligned options

[kp]

Summary

It turns out pf incorrectly updates the TCP checksum if the TCP option
we're modifying is not 2-byte algined with respect to the start of the
packet.

Create a TCP packet with such an option and throw it through a scrub
rule, which will update timestamps and modify the packet.

PR: 240416

[kp]

• Add MSS
• Also test the aligned case

[kp]

Committed as https://cgit.freebsd.org/src/commit/?id=2d3fda5fa1dc99aa8788e5f8d8bb71e682101063