pf tests: ensure temporary files end up in the atf working directory

Many of the tests create temporary files. pid files, log files, tcpdump
captures, ... We should take care to ensure they're stored in the temporary
working directory Kyua creates rather than in the root directory.

This ensures there are no conflicts between simultaneously running tests, and
also keeps the root directory clean.

MFC after: 1 month
Sponsored by: Rubicon Communications, LLC ("Netgate")