pf tests: make killstate tests more robust

Rather than using a Scapy-based Python script only check if the state
still exists. Scapy tends to be slow to start, it appears because it
lists all interfaces and gets their (IPv6) addresses a couple of times
at startup. This can be sufficient for the ICMP state to time out and
the test to fail.

We now only check if the state exists or is removed as expected, which
makes things faster, and should mean the test is more robust on slower
machines (such as CI VMs).

Sponsored by: Rubicon Communications, LLC ("Netgate")