pf tests: Make killstate:match more robust

The killstate:match test starts nc as a background process. There was no
guarantee that the nc process would have connected by the time we check
for states, so this test occasionally failed without good reason.

Teach the test to wait for at least some states to turn up before
executing the critical checks.

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