docs: add TESTING.md guide for contributors (resolves #9351)

[jayshilj]

docs: add TESTING.md guide for contributors (resolves #9351)

Resolves #9351

Problem

External contributors to dbt-core currently have no dedicated reference
for understanding what types of tests to add when submitting a PR. This
makes it harder for first-time contributors to know whether to write a unit
test or a functional test, and where to find existing examples to follow.

Solution

Added a new TESTING.md living document at the root of the repo that covers:

• When to write unit tests (tests/unit/) with links to existing examples
• When to write functional/integration tests (tests/functional/) with examples and considerations
• How to run each type of test locally using hatch and pytest

Also added a reference link to TESTING.md in the ### Unit, Integration, Functional?
section of CONTRIBUTING.md so contributors can discover it naturally.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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