Npm test Yeoman Custom Generator

some days ago i've started to write a custom yeoman generator and i want to make a test for it (so i can use travis ecc) but i have a problem when i lunch npm test. My generator copy only 4 files, a gulpfile.js and 3 files each in a different folder so the final result is:

Project-folder
|
|----- gulpfile.js
|--------production
             ---------css/main.css
             ---------html/index.html
             ---------js/index.js

I want to test my generator so i write this test,i start to write it from the template test provided by generator-generator

app.js

'use strict';
var path = require('path');
var assert = require('yeoman-assert');
var helpers = require('yeoman-test');

describe('generator-postcss-template:app', function () {
  before(function () {
    return helpers.run(path.join(__dirname, '../generators/app'))
      .withPrompts({someAnswer: true})
      .toPromise();
  });

  it('creates files', function () {
    assert.file([
      'production/css/styles.css',
      'production/html/index.html'
    ]);
  });
});

When i lunch the npm test i have this error

module.js:471
    throw err;
    ^

Error: Cannot find module 'gulp-eslint'
    at Function.Module._resolveFilename (module.js:469:15)
    at Function.Module._load (module.js:417:25)
    at Module.require (module.js:497:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/run/media/giulio/Lavoro/Personal/generator-postcss-template/gulpfile.js:4:14)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
npm ERR! Test failed.  See above for more details.

i don't know how fix it,test of generator-generator is the same but check only if dummyfile was created and test pass with 100%

Some one can help me?