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")