async-test

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
}

router\.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('my-route');
  this.route('your-route');
});

export default Router;

routes.my-route\.js

import Route from '@ember/routing/route';

export default Route.extend({
  // this causes the test to fail
  async beforeModel() {
    this.transitionTo('/your-route');
	}
  
  // this works
  //beforeModel() {
  //  this.transitionTo('/your-route');
	//}

});

templates.application\.hbs

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

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

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

module('TODO: put something here', function(hooks) {
  setupApplicationTest(hooks);

  test('visiting /my-route', async function(assert) {
    await visit('/my-route');

    assert.equal(currentURL(), '/your-route');
  });
});

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"
  }
}