pf tests: Stress state retrieval

Create and retrieve 20.000 states. There have been issues with nvlists
causing very slow state retrieval. We don't impose a specific limit on
the time required to retrieve the states, but do log it. In excessive
cases the Kyua timeout will fail this test.

Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30943