Add TestRenderer.act callback function

_I am..._

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| Requesting a new feature
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**Proposal/Feature-request:**

Add callback to `render` function to run code inside `TestRenderer.act` function. https://github.com/airbnb/react-sketchapp/blob/b238e69c6f1e65ec6b1d8a908a91fbd9a8cc43a7/src/buildTree.ts#L96-L99

This should allow for Relay data fetching to occur before the component is rendered: https://relay.dev/docs/guides/testing-relay-components/#examples

I’ll work on hacking together a demo for this with a mock GraphQL Relay API; this could be expanded to having a full Relay example in this repo.

Start at src/buildTree.ts around lines 96-99 and review how the render function is called. Check the React TestRenderer.act documentation and the linked Relay testing example; done means render accepts a callback that runs inside act so Relay data fetching can complete before rendering, with the behavior demonstrated using a mock Relay API.

Written by the indexing model from the issue text.