Fix chdir leaking on error out of cd-contextmanager of tests/helper.py.

[aknrdureegaesr]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• I tested my changes.

Description

While working on something else, I happened to spot a bug in tests/helper.py: The change of directory can (unintentionally) leak out of the cd context manager if the code inside the context throws an exception. In other words, on error, the change of directory is not undone.

I added a test for the test helper that demonstrates the bug if run with the previous test helper code. Not sure whether you want that.

This is a rather trivial change not visible to the end-user, so there is no need to add to CHANGELOG.

I plead guilty not having created a separate bug issue first. Do you need it in such a trivial case?

[Kwpolska]

I plead guilty not having created a separate bug issue first. Do you need it in such a trivial case?

Not really.

Thanks for fixing this!