pf tests: Test scrub fragment reassemble on interfaces with different MTU

There's a problem with pf's reassembly code where it produces incorrect
checksums when reassembling across interfaces with different MTUs.
Test this.

PR: 255432
Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30013

(cherry picked from commit 388c0cde10293d9a3434e99146bf391aec6878a3)