Its possible to create up to 100 test users per app, creation allows you to choose whether the user already should have the application installed and which permissions you want them to have, using API calls. You can also remove users, list all of your app's test users and even create friend connections between them using the API. This really is a great resource, but using an API to get this information all the time is cumbersome and might take time away from your time to develop the application itself. This is where the Test User Manager comes in.
He explains some of his thoughts behind the development structure of the tool and includes some basic descriptions (and screenshots) of listing out the test users for an application, making a new user and creating/viewing the relationships between those users. As of the writing of his post, he's in version 0.9 which you can download from github. Obviously, you can also clone the repository and submit your own enhancements too if you'd like.