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 wouldn't revert this, it doesn't hurt and it's useful for cases like mine. On the other side I can understand you, so hard to decide :)