pf tests: Stress state retrieval

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

(cherry picked from commit d8d43b2de1fa679179f7089cb3c31e6780ec82af)