add bullet point for testing in tutorial (#9248)

[bernard-lin]

#9248
Before submitting a pull request, please make sure the following is done:

1. Fork the repository and create your branch from master.
2. If you've added code that should be tested, add tests!
3. If you've changed APIs, update the documentation.
4. Ensure the test suite passes (npm test).
5. Make sure your code lints (npm run lint).
6. Format your code with prettier (npm run prettier).
7. Run the Flow typechecks (npm run flow).
8. If you added or removed any tests, run ./scripts/fiber/record-tests before submitting the pull request, and commit the resulting changes.
9. If you haven't already, complete the CLA.

[gaearon]

Hey, I’m very sorry for closing this since you acted on a contributor suggestion. Please see my reply in #9248 (comment) for the reason I’m closing the PR. I appreciate the effort!

[bernard-lin]

No worries! I appreciate the discussion you and Brandon are having in the other thread