pf tests: add missing atf_test_case lines for nat tests

The lack of these lines means we don't call the cleanup function.
That's not as bad as it could be, because these tests are nested in a jail by
Kyua, so most of the cleanup work (destroying jails and interfaces) is done when
the parent jail goes away, but it's still better to have these lines.

Sponsored by: Rubicon Communications, LLC ("Netgate")