Adding pytest runner and a tests badge to this repo.

[julianharty]

This is both an experiment - to try out genbadges for example and improvements as we'd like to use code quality tools, automated tests, and visible reporting.

Fixes #34.

Notes:

• The files in reports/junit/ are only committed here for ease of access to reports; they're not part of the active codebase.
• Trailing whitespace has been removed from the reports by the pre-commit checks.

[julianharty]

@tnzmnjm This has ended up as a much larger PR than expected after I added another commit that incorporated ruff into the pre-commit hooks together with the changes that it then made to reformat the code in the repo. Thankfully, the changes it made seem very clear and easy to follow (let me know if there's anything that looks wrong or doesn't make sense). See #36 for a summary of the ruff integration and how to run the pre-commit checks interactively.

FYI I'd intended to create a separate PR for the ruff integration but github decided to add the commit to this PR instead.