pf tests: check that we clean up unused kifs

The previous commit fixed a memory leak, where we'd fail to clean up
removed groups (and interfaces).
Check that we now clean those up as expected.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37570

(cherry picked from commit 654e8d84ec8bcb3d94cd8de69edecbe1bbd73eb8)