getApp.js

var app = require("express")();

app.get('/user', function(req, res){
  res.send(200, { name: 'marcus' });
});

// In order to reach the app from other modules
// we need to export the express application
module.exports.getApp = app;

gettest.js

var request = require('supertest');

// Here we get hold of the express application 
// by using the exported 'getApp'-property
var app = require("./getApp").getApp;

describe('GET /users', function(){
  it('respond with json', function(done){
    // the request-object is the supertest top level api
    request(app)
      .get('/user')
      .set('Accept', 'application/json')
      .expect('Content-Type', /json/)
      .expect(200, done); // note that we're passing the done as parameter to the expect
  });
});