Add automated tests

[simonw]

No description provided.

[simonw]

I got Claude 3.5 Sonnet to write the initial tests:

files-to-prompt . --ignore LICENSE | llm -m claude-3.5-sonnet -s 'step by step advice on how to implement automated tests for this, which is hard because the tests need to work within a temporary Django project that lives in the tests/ directory somehow. Provide all code at the end.'

Response: https://gist.github.com/simonw/a1c51e3a4f30d91eac4664ba84266ca1#response

Then I poked at them with the help of https://pytest-django.readthedocs.io/en/latest/managing_python_path.html until I got them to run. Then I modified them a bit to add some extra assertions.