Replace automatic test start via timeout with explicit start() call

[Turbo87]

Projects have to call start() in their test-helper.js file from now on:

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

setApplication(Application.create(config.APP));

start();

similar to ember-cli/ember-cli-qunit#176

Resolves #212