Due to nix-store making its paths read-only, pytest was unable to remove
the test files and hence the entire temporary directory, screaming all
over the place in stderr about that, getting worse for each test run.
By making the nix fixture first yield nix and then, after the test
finished running, changing the file permissions to include read on all
files and directories within the temp folder, pytest is able to properly
remove old test runs again
Additionally added more clear instructions for file deletion to the
pytest configuration
Change-Id: Ia7e3d195665968ac80a57d0e525691b28be7f503
Commentator2.0
5daddad39a