Allow test-helpers to accept DOM Elements

[sukima]

One of the issues I ran into I didn't have access to the selector strings but instead the actual DOM element. This is usually the case in addons like fractal-page-object.

With this change we can seamlessly use both. Ember test-helpers function with both a string selector or a DOM Element. Here is a working example:

class Foo extends PageObject {
  form = selector('[data-test-selector=my-form]', class extends PageObject {
    calendar = selector('ember-power-calendar', class extends PageObject {
      selectDate(date) {
        return calendarSelect(this.element, date);
      }
    });
  });
}