Back when I was working at Canonical on the Ubuntu phone project, test-driven development was the default and natural way to work. One thing I noticed and genuinely liked was how Ubuntu and Debian ship functional tests alongside many of their packages. Not only build-time unit tests hidden in the CI pipeline, but installable test binaries that you can run against the actual packages on an actual system. They are named *-tests or *-test, and they live in the archive next to the production packages.