tests: fix failing doctest in match.py by adding dummy auditor It was failing with "OSError: [Errno 13] Permission denied: '/root/sub'". It has been failing since 8b1a9ba375e5 (match: make sure `root` argument is always an absolute path (API), 2019-12-13). I don't know why I didn't notice it before sending that patch. Differential Revision: https://phab.mercurial-scm.org/D7731