[fix] stale testing.md link

[pq]

The link didn't seem to make the wiki migration:

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our [contributor guide](https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Dart contributions to our repos should follow the Dart style guide and use dart format.
• Java and Kotlin contributions should strive to follow Java and Kotlin best practices (discussion).