Add link to Vitest & Ava docs (#6983)

CONTRIBUTING.md

@@ -213,7 +213,10 @@ As a reviewer, it's important to be thoughtful about the proposed changes and co
 
 ## PR Tests
 
-Every PR should include tests for the functionality that's being added. Most changes will be to [Wrangler](packages/wrangler/src/__tests__) (using Vitest), [Miniflare](packages/miniflare/test) (using Ava), or [C3](packages/create-cloudflare/src/__tests__) (using Vitest), and should include unit tests within the testing harness of those packages.
+Every PR should include tests for the functionality that's being added. Most changes will be to [Wrangler](packages/wrangler/src/__tests__) (using Vitest), [Miniflare](packages/miniflare/test) (using Ava), or [C3](packages/create-cloudflare/src/__tests__) (using Vitest), and should include unit tests within the testing harness of those packages. For documentation on how these testing frameworks work, see:
+
+- Vitest: https://vitest.dev/guide
+- Ava: https://github.com/avajs/ava?tab=readme-ov-file#documentation
 
 If your PR includes functionality that's difficult to unit test, you can add a fixture test by creating a new package in the `fixtures/` folder. This allows for adding a test that requires a specific filesystem or worker setup (for instance, `fixtures/no-bundle-import` tests the interaction of Wrangler with a specific set of JS, WASM, text, and binary modules on the filesystem). When adding a fixture test, include a `vitest.config.mts` file within the new package, which will ensure it's run as part of the `workers-sdk` CI. You should merge your own configuration with the default config from the root of the repo.