dillon1000/react

Apply #20778 to new fork, too (#20782)

* Apply #20778 to new fork, too

* Fix tests that use runWithPriority

Where possible, I tried to rewrite in terms of an idiomatic API.

For DOM tests, we should be dispatching an event with the desired
priority level.

For Idle updates (very unstable feature), probably need an unstable
API like ReactDOM.unstable_IdleUpdates.

Some of these fixes are not great, but we can replace them once we've
landed the more of our planned changes to the layering between
Scheduler, the reconciler, and the renderer.