fix: 🐛 debug code depended on old unittest, updated for pytest

[martonvago]

Description

These changes fix failing tests by updating the way we check whether tests are running. This became out of date after switching to pytest. As a result, sample data was loaded into the test database, so tests checking if the Files table was empty were failing.

I've also added an extra test debug config allowing us to set breakpoints etc. in external files (e.g. library files or Django core code) -- feel free to ignore/change this.

Closes #523

Reviewer Focus

This PR needs a quick review.

Checklist

• [x] Added or updated tests
• [x] Tests passed locally
• [x] Linted and formatted code
• [x] Build passed locally
• [x] Updated documentation

[martonvago]

Thanks, it's not particularly urgent for me, so it can wait :)