Expand the output file handling on the testing documentation page

**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