Add one test fixture note for contributors

[rxdt]

Goal

Add a short note for contributors explaining where tests live and how to run the focused test suite.

Suggested scope

Mention harness/tests/ and the relevant pytest command from the project docs.

Acceptance criteria

• One short contributor-facing note.
• No test implementation required.
• Docs-only PR.

[boraiks]

Hi @rxdt! I'd like to take this as my first open-source contribution. I've confirmed the tests live in harness/tests/, and I've actually run both harness preflight and harness gate locally (WSL2 + uv) to verify the commands work. My plan: add a short, contributor-facing note pointing to harness/tests/ and these commands (rather than calling pytest directly), kept accurate against the repo per the docs-only guidance.
One question: would you prefer the note in the README or added to the "Checks" section of CONTRIBUTING.md?
Also, a small heads-up unrelated to this issue: running the gate surfaced 23 ruff rule-codes-in-selectors warnings in pyproject.toml (rule codes used instead of names, all auto-fixable). Happy to open a separate PR for that if it'd be useful. Could you assign this one to me?

[rxdt]

Adding this to the CONTRIBUTING.md would be perfect. A separate PR would be great. Thanks for your help!

I just came across the Ruff rule-code-selector warnings myself. Luckily ruff check --fix auto-fixed that. (Ruff added a new rule this past week that some rules must be explicitly named)

[boraiks]

Quick follow-up on the ruff rule-code warnings I mentioned: I pulled the latest main and they're already resolved, so no separate PR needed there.