In node I use simple test libraries like tap or tape that let you run the test files directly. For code that needs to run in both the browser and node I use tape because tap doesn't run in the browser very well and the APIs are mostly interchangeable.
The simplest kind of test I might write in test/ looks like:
var test = require('tape');
var someModule = require('../');
test('fibwibblers and xyrscawlers', function (t) {
t.plan(2);