Interesting! The Exercism website is really impressive—congrats to @kytrinyx.
I've been doing a lot of automated feedback stuff with my students too, but they're more focused on HTML/CSS. I tried TravisCI for a short time but it became too slow for the amount & complexity of tests that ran (like HTML & CSS validator). So I switched to an Electron app that they can run on their desktops. The feedback from the students has been absolutely fantastic!