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

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
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27688