App-test.js

/* global jest, expect */

jest.dontMock('../App');
jest.dontMock('../../decorators/decorator');

const React = require('react');
const ReactDOM = require('react-dom');
const ReactTestUtils = require('react-addons-test-utils');
const App = require('../App');

describe('App', () => {
  it('should set state', () => {
    const component = ReactTestUtils.renderIntoDocument(<App />);
    expect(ReactDOM.findDOMNode(component).innerHTML).toEqual('ほげほげふがふが');
  });
});

App.js

import React, {PropTypes, Component} from 'react';
import decorator from '../../decorators/decorator';

@decorator()
class App extends Component {
  render() {
    return <div>ほげほげふがふが</div>;
  }
}

export default App;

preprocessor.js

/* eslint-disable */

'use strict';

var babel = require('babel-core');

module.exports = {
  process: function(src, filename) {
    if (!babel.canCompile(filename)) {
      return '';
    }
    if (filename.indexOf('node_modules') === -1) {
      return babel.transform(src, {
        filename: filename,
        retainLines: true,
        auxiliaryCommentBefore: 'istanbul ignore next'
      }).code;
    }
    return src;
  }
};