async_test.js

var sinon = require('sinon');
var assert = require('assert');

function async(cb) {
  setTimeout(function() {
    cb();
  }, 1000);
}

describe('async()', function() {
  it('should call callback', function() {
    var clock = sinon.useFakeTimers('setTimeout');
    var cb = sinon.spy();
    async(cb);
    clock.tick(999999);
    assert(cb.called);
  });
});

async_test_invalid.js

var sinon = require('sinon');
var assert = require('assert');
var http = require('http');
var nock = require('nock');

function async(cb) {
  http.request({ hostname: 'localhost', path: '/' }, function(res) {
    res.on('end', function() {
      cb();
    });
  }).end();
}

describe('async()', function() {
  it('should call callback', function(done) {
    var clock = sinon.useFakeTimers('setTimeout');
    var api = nock('http://localhost')
          .get('/')
          .reply(200);
    var cb = sinon.spy();
    async(cb);
    clock.tick(999999);
    assert(cb.called);
  });
});