We do support that in our Classroom product (https://repl.it/classroom) so it'd be pretty easy to expose that in our repl interface. Might be able to knock this out in an afternoon. (For NodeJS developers we support Jest out of the box: https://repl.it/@amasad/try-jest).
This example always runs the tests when you hit cmd-enter. I was wondering how you would have a running program with tests.
I looked in https://repl.it/ibuiltthis but couldn't easily find examples of NodeJS projects. Could you point me at one? (Feel free to point me at a better forum for these noob questions.)
Fullstack apps support was just released to do so not many examples on our forum. If you go to https://repl.it/languages/express you'll see a simple scaffold for ExpressJS.