docs: update django integration testing guide

[crypto-rizzo]

Closes #

Successful PR Checklist:

• [ ] Tests
  • [ ] (not applicable?)
• [ ] Documentation
  • [ ] (not applicable?)

PR label(s):

• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20breaking%20%F0%9F%92%A5
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20feature%20%E2%AD%90%EF%B8%8F
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20bugfix%20%F0%9F%95%B5%EF%B8%8F
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• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20documentation%20%F0%9F%93%9A

[github-actions[bot]]

Coverage report

This PR does not seem to contain any modification to coverable code.

[ewjoachim]

I've added a commit in #1058, but I can't add it to this PR directly. Thank you very much :)

[crypto-rizzo]

Of course! Didn't think this also applied to PyTest, good catch / add here 👍