[8.x] Notification assertions respect shouldSend method on notification

[romalytvynenko]

Support for notification's shouldSend method were added in PR #37930. This method allows users to add some logic to cancel sending notification based on given notifiable and channel.

Problem

However, testing methods assertSentTo and assertNotSentTo weren't aware about this feature and didn't behave as expected (reported here as well #38909).

For example, take this code:

// somewhere in user's code
Notification::send($user, new SomeNotification);

// implementation
class SomeNotification extends Notification
{
    public function via($notifiable)
    {
        return ['mail'];
    }

    public function shouldSend($notifiable, $channel)
    {
        return false;
    }
}

Now, when testing whether notification hasn't been sent, we won't get expected assertion result:

Notification::fake();

// some code triggering notification send

Notification::assertNotSentTo($user, SomeNotification::class); // will fail, but should pass

Solution

This PR adds support of shouldSend to notification's assertSentTo and assertNotSentTo methods. So now when shouldSend is presented on notification class and returns false for some notifiable, assertSentTo and assertNotSentTo can be used to check that behavior.

Fixes #38909