Do you need to have a new content type under Content menu along with the Testimonial? If so, I would recommend you to create a new module using the Module builder
and add related data fields for that source content type.
From other hand, attached is the amended Module (including Globa.asax) with added additional Source field, where you could check all the changes made in the files for adding the field in the frontend and the backend for view/edit/create.
In order to test the sample new implementation, you need to drop the current database in advance.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items