Allow #ember-testing-container to be optional.
Allow #ember-testing-container to be optional.

QUnit runs tests just fine without any modifications to the DOM. This change is important for running tests that require the application being set on the `<body>` element (scrolling-related in particular) or have CSS that is impacted by the modified application structure.
ember-cli/ember-cli-qunit,ember-cli/ember-cli-qunit,nathanhammond/ember-cli-qunit,nathanhammond/ember-cli-qunit