pf tests: Test clearing rules counters

This was briefly broken, so ensure that we can read and clear rules
counters.

MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29728

(cherry picked from commit 112f007e12818ba3d54869b331139090e69dbc74)