[9.x] Add NotificationFake::assertNothingSentTo()

[axlon]

Right now there is no easy way to assert that no notifications have been sent to a specific notifiable. Laravel does allow you to assert that no notifications have been sent at all, but this isn't always applicable.

This is primarily an issue when a piece of code dispatches various notification types to different notifiables, for example:

$uploader = User::factory()->createOne();
$podcast = Podcast::factory()->for($uploader)->createOne();
$episode = PodcastEpisode::factory()->for($podcast)->createOne();

$subscriber = User::factory()->hasAttached($podcast)->createOne();
$randomUser = User::factory()->createOne();

Bus::dispatchSync(new ProcessPodcast($episode));

Notification::assertSentTo($uploader, EpisodeProcessed::class);
Notification::assertSentTo($subscriber, NewEpisodeAvailable::class);

Notification::assertNotSentTo($randomUser, EpisodeProcessed::class);
Notification::assertNotSentTo($randomUser, NewEpisodeAvailable::class);

The example above is quite simple and only sends 2 notifications, but the number of calls to assertNotSentTo would increase by one for every additional notification type that is dispatched.

This PR aims to simplify testing in these kinds of scenario's by adding Notification::assertNothingSentTo($notifiable)

PS. This PR includes a test for NotificationFake::assertNothingSent(), which appeared to be missing