Expand the output file handling on the testing documentation page

[arim97]

Your Godot version:
4.2.2
Issue description:
Learning how to do tests for Godot development, and had a very tough time handling and troubleshooting output files to get desired results. Whole section feels disorganized compared to previous section on writing tests, which is very helpful.
URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/unit_testing.html