1-service.js

/**
 * app/services/storage.js
 */

import Service from 'ember-service';
import computed from 'ember-computed';
import Ember from 'ember';

const { getOwner } = Ember;

export default Service.extend({
  engine: computed(function() {
    let owner = getOwner(this);
    
    return owner.lookup('engine:storage');
  })
});

2-initializer.js

/**
 * app/initializers/storage-engine.js
 */

export function initialize(application) {
  let engine = application.storageEngine || window.localStorage;
  
  application.register('engine:storage', engine, { instantiate: false });
}

export default {
  name: 'storage-engine',
  initialize
};

3-test-helper.js

/**
 * tests/helpers/module-for-acceptance.js
 */

import { module } from 'qunit';
import Ember from 'ember';
import startApp from '../helpers/start-app';
import destroyApp from '../helpers/destroy-app';
import MockStorage from 'my-app/tests/helpers/mocks/storage-engine';

const { RSVP: { Promise } } = Ember;

export default function(name, options = {}) {
  module(name, {
    beforeEach() {
      let storageEngine = new MockStorage();
      
      this.application = startApp({ storageEngine });
      
      this.storageEngine = storageEngine;

      if (options.beforeEach) {
        return options.beforeEach.apply(this, arguments);
      }
    },

    afterEach() {
      let afterEach = options.afterEach && options.afterEach.apply(this, arguments);
      return Promise.resolve(afterEach).then(() => destroyApp(this.application));
    }
  });
}