pf tests: pfsync bulk update test

Test that pfsync works as expected with bulk updates. That is, create
some state before setting up the second firewall. Let that firewall
request a bulk update so it can catch up, and check that it got the
state which was created before it enable pfsync.

PR: 254236
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29272