pf tests: make killstate:match more robust

Only lists the states relevant to the connection we're killing.
Sometimes there are IPv6 related states (due to the usual IPv6
background traffic of router solicitations, DAD, ...) that causes us to
think we failed to kill the state, which in turn caused the test to fail
intermittently.

MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 4e860bd5da1423aac9aed0541c484ebf9c1d1621)