[9.x] Add helper to dispatch fake job batches

[jasonmccreary]

This adds a helper method to the BusFake for quickly dispatching a fake job batch with no jobs for testing purpose. It's similar to the new Job helper added in #44104, but when you want to test a batch independently.

Before:

Bus::fake();

$batch = Bus::batch([])->dispatch();
Cache::put('batched-automation', $batch->id);

Shift::runAutomation('v9.31.0'); // cancels previous batched job

Bus::assertDispatched(UpdateRepositories::class);
$this->assertTrue($batch->cancelled());

After

Bus::fake();

$batch = Bus::dispatchFakeBatch();
// ...

For convenience of testing, you may optionally pass the name of the batch. If your test requires more detail, you may continue to create and dispatch a batch with the longhand methods.