Edit Nov 2016: Node now has a built in debugger that you can start with --inspect. This answer explains it: https://stackoverflow.com/a/39901169/30946.
None of the existing answers mention the path of least resistance: When you need to debug Mocha tests, you can simply add another assert that checks the value you'd like to debug.
myVar.should.equal(expected);
I find this is often all I need. And I just remove the extra assert(s) I used for debugging when I'm done.