pfctl tests: test case for the POM_STICKYADDRESS fix

In 1e73fbd8b we fixed an issue with POM_STICKYADDRESS being checked in
the wrong struct.
Add a basic test case for this fix.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 63167eb48c9f275384bae20e9b432ad68f44ff42)