[Mock Scheduler] Mimic browser's advanceTime (#17967)

The mock Scheduler that we use in our tests has its own fake timer
implementation. The `unstable_advanceTime` method advances the timeline.

Currently, a call to `unstable_advanceTime` will also flush any pending
expired work. But that's not how it works in the browser: when a timer
fires, the corresponding task is added to the Scheduler queue. However,
we will still wait until the next message event before flushing it.

This commit changes `unstable_advanceTime` to more closely resemble the
browser behavior, by removing the automatic flushing of expired work.

```js
// Before this commit
Scheduler.unstable_advanceTime(ms);

// Equivalent behavior after this commit
Scheduler.unstable_advanceTime(ms);
Scheduler.unstable_flushExpired();
```

The general principle is to prefer separate APIs for scheduling tasks
and flushing them.

This change does not affect any public APIs. `unstable_advanceTime` is
only used by our own test suite. It is not used by `act`.

However, we may need to update tests in www, like Relay's.