ember-data async testing issue

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

models.nurture.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { hasMany } from "ember-data/relationships";

export default Model.extend({
  name: attr('string'),
  sources: hasMany('source', { async: false })
});

models.source.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo } from "ember-data/relationships";

export default Model.extend({
	name: attr('string'),
	nurture: belongsTo('nurture', { async: false })
});

serializers.nurture.js

import RESTSerializer from 'ember-data/serializers/rest';
import EmbeddedRecordsMixin from 'ember-data/serializers/embedded-records-mixin';

export default RESTSerializer.extend(EmbeddedRecordsMixin, {
  attrs: {
    sources: { embedded: 'always' }
  }
});

serializers.source.js

import RESTSerializer from 'ember-data/serializers/rest';
import EmbeddedRecordsMixin from 'ember-data/serializers/embedded-records-mixin';

export default RESTSerializer.extend(EmbeddedRecordsMixin, {
  attrs: {
		name: { serialize: false }
	}
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

tests.helpers.destroy-app.js

import Ember from 'ember';

export default function destroyApp(application) {
  Ember.run(application, 'destroy');
}

tests.helpers.resolver.js

import Resolver from '../../resolver';
import config from '../../config/environment';

const resolver = Resolver.create();

resolver.namespace = {
  modulePrefix: config.modulePrefix,
  podModulePrefix: config.podModulePrefix
};

export default resolver;

tests.helpers.start-app.js

import Ember from 'ember';
import Application from '../../app';
import config from '../../config/environment';

const { run } = Ember;
const assign = Ember.assign || Ember.merge;

export default function startApp(attrs) {
  let application;

  let attributes = assign({rootElement: "#test-root"}, config.APP);
  attributes = assign(attributes, attrs); // use defaults, but you can override;

  run(() => {
    application = Application.create(attributes);
    application.setupForTesting();
    application.injectTestHelpers();
  });

  return application;
}

tests.test-helper.js

import resolver from './helpers/resolver';
import {
  setResolver
} from 'ember-qunit';

setResolver(resolver);

tests.unit.serializers.source-test.js

import { moduleForModel, test } from 'ember-qunit';

moduleForModel('source', 'Unit | Serializer | source', {
  beforeEach: function() {
		this.server = new Pretender(function() {
			this.get('/sources', function() {
				let sources = [{
          id: 1,
          name: 'test source',
          nurture_id: 1
        }];
        let response = { sources: sources };
				return [200, { "Content-Type": "application/json" }, JSON.stringify(response)];
			});
			this.get('/nurtures/:id', function(request) {
				let nurture = {
          id: 1,
          name: 'test workflow',
          sources: [{
            id: 1,
            name: 'test source',
            nurture_id: 1
          }]
        }
				let response = { nurture: nurture };
				return [200, { "Content-Type": "application/json" }, JSON.stringify(response)];
			});
		});
	},
	afterEach: function() {
		this.server.shutdown();
  },
  needs: [ 'serializer:source', 'model:nurture', 'serializer:nurture']
});

// Replace this with your real tests.
test('it exists', function(assert) {
  let controller = this.subject();
  assert.ok(controller);
});

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": true
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.1",
    "ember-template-compiler": "2.9.1",
    "ember-data": "2.11.3",
    "ember-testing": "2.9.1",
    "FakeXMLHttpRequest": "https://rawgit.com/pretenderjs/FakeXMLHttpRequest/23c3a96b5b24f1bfe595867437e4f128a29c2840/fake_xml_http_request.js",
    "pretender": "https://rawgit.com/pretenderjs/pretender/c6de9afe18b1472aded2592f5a80ad9a26a0e262/pretender.js"
  },
  "addons": {
  }
}