[10.x] Support asserting against chained batches

[taylorotwell]

This improves the testability of batches that exist within chains via a new Bus::chainedBatch testing helper which allows you to generate a truth test for a batch within a chain.

Given this route:

Route::get('/batch', function () {
    Bus::chain([
        new TestJob,
        Bus::batch([
            new TestJob,
        ]),
        new TestJob,
    ])->dispatch();
});

A test could now be written like so:

public function test_the_application_returns_a_successful_response(): void
{
    Bus::fake();

    $response = $this->get('/batch');

    $response->assertSuccessful();

    Bus::assertChained([
        new TestJob,
        Bus::chainedBatch(function ($batch) {
            return $batch->jobs->count() === 1;
        }),
        new TestJob,
    ]);
}