Mock component in acceptance test

components.my-component\.js

import Component from '@glimmer/component';

export default class extends Component {
  get output() {
    return 'foo'; 
  }
}

templates.application\.hbs

<MyComponent />

templates.components.my-component\.hbs

<div class="my-component">
  {{this.output}}
</div>

tests.acceptance.my-acceptance-test\.js

import { module, test } from 'qunit';
import { visit, find, currentURL } from '@ember/test-helpers';
import { setupApplicationTest } from 'ember-qunit';
import MyComponent from 'app/components/my-component';

module('my-component on the homepage', function(hooks) {
  setupApplicationTest(hooks);
  
  class MockComponent extends MyComponent {
    // comment out this to make the test fail
    get output() {
      return 'bar'; 
    }
  }
  
  hooks.beforeEach(function() {
    this.owner.register('component:my-component', MockComponent);
  });

  test('output', async function(assert) {
    await visit('/');

    // assert.equal(find('.my-component').innerText, 'foo');
    
    assert.equal(find('.my-component').innerText, 'bar');
  });
});

tests.test-helper\.js

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

let attributes = {
  rootElement: '#test-root',
  autoboot: false
};
attributes = assign(attributes, config.APP);

let application = Application.create(attributes);
setApplication(application);

start();

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": true
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}