Call to a member function sess_save() on a non-object

I ended up making a flat file session driver that is loaded when my tests are being run 
Session_codeception.php
https://gist.github.com/withremote/99c0ad36999da43fb127

Then I made a MY_Session.php override which only loads this driver if it detects i'm running my tests.
MY_Session.php 
https://gist.github.com/withremote/7b7c8fed47f39524c0ba

Dirty, but it worked. I didn't need to test that session were being stored, just that the session driver was available to my libraries when running tests.