I'm still experimenting with UIs for live (sandboxed) evaluation of tests. I've realised that you really want to highlight the failing assertion, not just the failing test.
Feedback welcome :)
Deno is an interesting sandboxed Typescript VM that uses URLs for dependencies: https://deno.land/
It's created by Ryan Dahl (who created node.js) and tries to fix what he sees as design issues/complexities in node.