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
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