→ npmjs.com/package/nock

npm install nock –save-dev

// tools-spec.js
(...)
var nock = require("nock);
(...)
describe("loadWiki()", function() {
  before(function() {
    nock("https://en.wikipedia.org")
      .get("/wiki/Abraham_Lincoln")
      .reply(200, "Mock Abraham Lincoln Page");
  (...)
    expect(html).to.equal("Mock Abraham Lincoln Page");
  (...)
});