Implement better async act detection

[bcarroll22]

Describe the feature you'd like:

As implemented by @threepointone in testing-library/react-testing-library#407, more can be done to check if the async version of act is available. We'll probably need to implement this here as well, even though there's no version of React Native yet that officially supports React 16.9.

Suggested implementation:

I'd imagine most of what's in the PR i referenced will work here, but I also don't know and async act (kinda act in general) is a bit of a mystery to me. Based off of a very high level overview of the PR, I think the tests would probably pass as well, assuming we get act from the test renderer instead of ReactTestUtils. But I'm not positive.

Describe alternatives you've considered:

We could always just not do this since no version of RN supports it and there haven't been any bug reports because of the current patch for those who already use the alpha in their apps.

Teachability, Documentation, Adoption, Migration Strategy:

Won't change anything here.

[bcarroll22]

🎉 This issue has been resolved in version 4.0.8 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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