[10.x] Add Notification testing helpers

[xurshudyan]

This pull request introduces a new testing helpers, specifically focusing on detecting if a notification has been queued. Currently, Laravel provides extensive testing helpers for notifications, but lacks a direct method to determine if a notification has been queued.

class InvoicePaid extends Notification implements ShouldQueue
{
    use Queueable;
}


$user = User::first();

$user->notify(new InvoicePaid());

Notification::assertNothingQueued();

Notification::assertQueued($user, InvoicePaid::class);

Notification::assertQueuedCount(1);

Notification::assertNotQueued($user, OrderShippedNotification::class);

[taylorotwell]

I'm not sure we really need this atm. You can assert that the notification was sent. If you want to see if it is queued you can just assert that the notification implements ShouldQueue.