Implement startApplicationTest from emberjs/rfcs#268.

[rwjblue]

Implements the startApplicationTest API proposed by emberjs/rfcs#268.

[rwjblue]

Once this lands, normal application acceptance tests will look like (demo uses qunit-dom because its awesome):

import { module, test } from 'qunit';
import { visit, click } from '@ember/test-helpers';
import { setupApplicationTest } from 'ember-qunit';

module('my awesome feature', function(hooks) {
  setupApplicationTest(hooks);

  test('can click on the button', async function(assert) {
    await visit('/whatever/route');
    
    assert.dom('.counter').hasText('0');

    await click('.increment-counter');

    assert.dom('.counter').hasText('1');
  });
});