Internal test helpers: Use Node's MessageChannel to queue task (#26345)

To wait for the microtask queue to empty, our internal test helpers
schedule an arbitrary task using `setImmediate`. It doesn't matter what
kind of task it is, only that it's a separate task from the current one,
because by the time it fires, the microtasks for the current event will
have already been processed.

The issue with `setImmediate` is that Jest mocks it. Which can lead to
weird behavior.

I've changed it to instead use a message event, via the MessageChannel
implementation exposed by the `node:worker_threads` module.

We should consider doing this in the public implementation of `act`,
too.