Don't shift interleaved updates to separate lane

[acdlite]

Now that interleaved updates are added to a special queue, we no longer need to shift them into their own lane. We can add to a lane that's already in the middle of rendering without risk of tearing.

See #20615 for more background.

I've only changed this in the new fork, and only behind the enableTransitionEntanglements flag.

Most of this commit involves updating tests. The "shift-to-a-new" lane trick was intentionally used in a handful of tests where two or more updates need to be scheduled in different lanes. Most of these tests were written before startTransition existed, and all of them were written before transitions were assigned arbitrary lanes.

So I ported these tests to use startTransition instead, which is the idiomatic way to mark an update as parallel.

I didn't change the old fork at all. Writing these tests in such a way that they also pass in the old fork actually revealed a few flaws in the current implementation regarding interrupting a suspended refresh transition early, which is a good reminder that we should be writing our tests using idiomatic patterns as much as we possibly can.