Unit testing: document necessity of being in Godot's root folder

**Your Godot version:**

4.7 Stable (Applies to any Godot version).

**Issue description:**

In order for tests to be run correctly, the command launching Godot with the test option `--test` must be executed with the Godot source repository root as the working directory.

The documentation does instruct to execute `./bin/ --test` in the "Running tests" section. I feel like it's not clear that this is a hard requirement and not just a way to keep the docs lean by skipping CD'ing into `/bin` after executing `scons` from the root folder.

Further down, the "Integration tests for GDScript" does instruct explicitly to CD the root folder before doing further operations. Since this is in the context of writing new GDScript tests, I don't feel like it would be automatic to assume this is a requirement for running them as well. Furthermore, if a user has a problem generically running the tests, they're going to look for help in the "Running tests" section rather than the one about writing new tests specifically for GDScript.

Because of this, I think there should be a a small sentence or perhaps a WARNING or NOTE box (like the one already present for information about `dev_mode`) making it explicit that this is a hard requirement.

**URL to the documentation page (if already existing):**

[Unit testing](https://docs.godotengine.org/en/stable/engine_details/architecture/unit_testing.html)

Start with the Unit testing documentation page, especially the “Running tests” section and its existing dev_mode note. Make the working-directory requirement explicit there, then verify the instructions clearly say that Godot tests must be launched from the repository root.

Written by the indexing model from the issue text.