This makes it easier to run a large collection of tests, compared to assert_cmd.
The design is inspired by trybuild with thought given to how mdBook books could pull in content so you can verify a code sample, the command for running it, and the output. In considering how to keep clap's website up-to-date, I had this idea and threw it together to see how well it works. Overall, seems good with room for improvement. I'll have to give this a try on a real world program soon.