[5.5] Allow testing anonymous notifiables

[themsaid]

Storing the notifiable in the fake notifications array will give us the change to do something like:

Notification::assertSentTo(new AnonymousNotifiable(), Notification::class, function($notification, $channels, $notifiable){
            return $notifiable->routes['mail'] == 'mail@yahoo.com';
        });

[antonkomarev]

Thank you @themsaid ! I've tried to write test just yesterday and understand that it's impossible. Was thought to create issue in internals.