That is an interesting idea, making an app that can test apps.
It's possible. But I don't think it's any easier than setting up testing directly in your app.
How would you set up tests? It would be impossible to set up generalized tests for every possible type of "code". You'd probably still need
*.test.js files with custom assertions.
I'd love to be proven wrong though. Imagine how many hours could be saved if there were a completely automated test "app".