tests: run the whole hg-core/path_auditor test in a clean temp dir This makes the whole test happen in a clean temporary directory in `$TMPDIR/$random`, and simplifies the test a bit by eliminating unnecessarily dynamic path elements computations. Before this patch, the first part of the test was happening in `/tmp` itself. This allowed coincidentally named files placed in that directory to impact the outcome of the test. Additionally, this made the second part of the test fail on systems on which `$TMPDIR != /tmp`, because the inspected directory was different from the one in which the mock files were being written. This fully fixes the issue only partially solved in db2bc9e667a1. Differential Revision: https://phab.mercurial-scm.org/D11738