Adds an unmount check to AnimatePresence prior to animation exit callback

[ctrlplusb]

I was getting issues in my jest tests without this, due to state updates occurring on an unmounted component. My component under test had an exit animation that completed post unmount of the parent component.

I also fixed the tests that were causing console.error logs. Refactored them to use the "async act" test util. IMO, the existing tests need a few more cleanup / refactor rounds to utilize this same strategy. Unfortunately I don't have the capacity to help with that right now. I did at least refactor the tests that were producing console.error logs. 👍

[codesandbox-ci]

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Sandbox	Source
Framer Motion: Simple animation	Configuration
App Store UI using React and Framer Motion	Configuration
Framer Motion: Reorder animation	Configuration
Framer Motion: growing item positionTransition issue	Configuration
Framer Motion: Image lightbox	Configuration

[mattgperry]

Sorry for the substantial delay, and thanks for submitting the PR!