Update README to reflect Ember 3.x test API

[jacobq]

I think the Overriding route-action for integration tests section of the README.md file might need to be updated because this.container appears to undefined inside of module --> hooks (Ember v3.0.0, Ember Data v3.0.1, jQuery v3.3.1). Instead, I found that using this.owner.__registry__ or this.owner.__container__.registry seems to work.

[knownasilya]

I created this helper:

import { helper } from '@ember/component/helper';

export default function overrideRouteActionsHelper(hooks, actions) {
  hooks.beforeEach(function () {
    this.owner.__registry__
      .registrations['helper:route-action'] = helper((arg) => {
        return this.routeActions[arg];
      });

    this.routeActions = actions;
  });
}

Use like so:

import overrideRouteActions from '../../../helpers/override-route-actions';

module('Integration | Component | table-mapping', function (hooks) {
  setupRenderingTest(hooks);
  overrideRouteActions(hooks, {
    matchBy() {}
  });
 //...
});

Where {{route-action 'matchBy'}} is in my component.

[knownasilya]

I could submit a PR if this is a good solution, and it could just be imported from this addon.