Proper Session Test

Maybe to close but especially to show a non-working example without `self::start()` in `CakeSession::delete()`. If you remove `self::start()` you will see an error occurres.

Refs: https://github.com/cakephp/cakephp/pull/4405
I reverted your patch. Thank you for your patience to sort this out and continued interested in helping to improve CakePHP.