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
The session is unset in `tearDown()` so at this point key `Delete.me` doesn't exist in session so trying to delete it will return false. This assert is incorrect.