[testing-library__react] Update cleanup to return Promise

[timswalling]

Updates types @types/testing-library__react to reflect changes introduced in testing-library/react-testing-library#519

---

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[typescript-bot]

@timswalling Thank you for submitting this PR!

🔔 @alexkrolick @kentcdodds @eps1lon @weyert @ifiokjr @danieljcafonso - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[eps1lon]

Although this is already covered by #43108 I approve to improve chances of getting a faster merge for v10. #43108 needs to be rebased anyway.

[typescript-bot]

A definition owner has approved this PR ⭐️. A maintainer will merge this PR shortly. If it shouldn't be merged yet, please leave a comment saying so and we'll wait. Thank you for your contribution to DefinitelyTyped!