The problem you are experiencing here will be due to these units being outdated and you not being able to access the old stack that was required for the tests to run but co incidentally we are in the process of updating them at the moment.
The first 3 units in the course have been updated just now (the rest to follow asap) to now run on the new Generation2 boxes so delete what you have done and then clone again, and then import into your Codio account (selecting the Empty Stack for the project) and all should run fine.
As to digging into the files in the project that make this all work, I would recommend you open guides in Edit mode so you will be able to navigate into the .guides folder to find any files you want and also as you view the guides in edit mode you will see the name of the assessment that was created, and within that, the references to the files being used.
Do also check out the Example Project mentioned on How to create auto-graded assessments:as this will also help
UPDATE: All Units in this course are now updated