New Twiddle

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
});

controllers.application.js

import Ember from 'ember';
import { getOwner } from '@ember/application';
import hbs from 'htmlbars-inline-precompile';
import MyComponent from '../components/my-component';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  init() {
    this._super(...arguments);
    getOwner(this).register('component:my-component', MyComponent.extend({
      layout: hbs`<h1>My mocked component</h1>`
    });
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{my-component}}
{{outlet}}
<br>
<br>

templates.components.my-component.hbs

<h1>My component's template</h1>

tests.test-helper.js

import Application from '../app';
import config from '../config/environment';
import { setApplication } from '@ember/test-helpers';
import { start } from 'ember-qunit';
import { assign } from '@ember/polyfills';

let attributes = assign({ rootElement: '#main' }, config.APP);
setApplication(Application.create(attributes));

start();

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}