waitForSyncStorage() doesn't wait for sync storage

`tests/e2e/utils/collaborative.ts:222`:

```ts
await page.waitForFunction(() => {
	return window.wp?.data?.select('core/editor')?.getCurrentPost() !== null;
});
```

That resolves as soon as the editor has a post, which happens well before the storage provider is wired up. Every collaborative test calling this is racing and passing for the wrong reason. It should be waiting on the provider itself.
