[6.x] Add assertDeleted for database testing

[jasonmccreary]

While there are numerous ways to test a record has been deleted, all are relatively lengthy to assert and don't communicate well.

Before

$this->assertDatabaseMissing('posts', [
    'id' => $post->id
]);

There is an assertSoftDeleted which can actually be passed a model. Upon doing so, it conveniently builds the appropriate expectations.

An assertDeleted method is symmetrical with this existing method and improves the developer experience by wrapping all the same behavior of assertDatabaseMissing in an intently named method.

After

$this->assertDeleted($post);

[jasonmccreary]

If merged, glad to add a note to the docs about the new method and optionally passing a model argument.