Implement files_at_path, directories_at_path, and stringify + write tests

The convention is to declare test variables in `let` blocks (and use `before :each` blocks to do initialization work that are not related to a particular variable).

The catch of using a closure of variables like this is that all the enclosed examples will use the same copy of test data because these initialization code are only ran once and are captured by the closure. Using `let` and `before` ensures they are freshly created before each example.

Also, it seems that the factories aren't being used here to create repos.