This is for unit-testing in V8 only: no DOM. I'm currently using PhantomJS, which is V8 + Webkit and seems to be a superset of this new "JS Test". I can't see any reason to add JS Test in addition to PhantomJS.
For several years I have been using QtWebKit to write tests that utilize the dom. Fast startup, console output etc. I wouldn't be surprised if the Chrome guys have a similar stripped down Chrome for JS unit tests.
Another possibility, which I use for testing my Firefox Add-ons, is using Rhino + env.js, which creates a very good simulation of the DOM.
Could be very helpful as a node.js addon
Not really, node.js already has heaps of testing frameworks that work fine in it.
non-dom based javascript, would be my first thought.