Trial includes a system called
trial-workbench which loads a bunch of commonly used dependencies, and then a file called
workbench.lisp. The file usually contains a setup that shows off some of Trial's capabilities, or in the case of a PR, a specific demo related to the feature being implemented.
The workbench is meant as a simple playground you can use to try things out with Trial. Many a feature or effect have been created using it, and it can be a good debugging aid to give a very minimal base case to test stuff against.
Do note that the workbench also uses the trial-assets repository to provide sample data. You can make use of those samples in your own projects as well.