Allows us to make assertions on the values that are yielded when
performing work. In our existing tests, we do this manually by pushing
into an array.

ReactNoop.flushThrough extends this concept. It accepts an array of
expected values and flushes until those values are yielded.

There's no advantage to scheduling updates with animation priority
versus scheduling sync work inside requestAnimationCallback. So we can
remove all the animation-specific code. Now there's only one type of
callback.

Behavior now matches Stack. It's unfortunate that this prevents us
from unifying SynchronousPriority and TaskPriority.