[5.7] NotificationFake can assert preferred locale

[derekmd]

When a notifiable implements the HasLocalePreference interface, NotificationFake should make the expected locale available for assertions.

e.g.,

Notification::fake();

$user = factory(User::class)->create(['locale' => 'au']);

// Call an artisan console command that invokes:
Notification::send($user, new AFLTrade('Chad Wingard', 'Hawthorn'));

Notification::assertSentTo($user, AFLTrade::class, function ($notification, $channels, $notifiable, $locale) use ($user) {
    return $notification->player === 'Chad Wingard' &&
        $notification->club === 'Hawthorn' &&
        $notifiable->is($user) &&
        $locale === 'au';
});

MailFake already supports similar locale assertion capabilities so I added a test for that too.