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
In general checking this issue in unit tests is not easy to due non-existence of session cookie, which is what `_checkSession()` check for. Can you provide some basic controller code to reproduce the issue?