tests: use tmp_path fixture to isolate files

[BoboTiG]

Lots of changes, but at the end it's a cleaner way to use a custom folder for each test. pytest will handle the auto-deletion of the temporary folder at the end of the test, no need to do it ourselves.

Going a step further, it is now useless to use force_overwrite=True when creating databases. But to ease the review, and eventual approval, I decided to not touch that part.

[mkrd]

@BoboTiG That's a lot cleaner, thanks for your contribution 👍