Add comments about the test setup steps.

As requested by @JulianLeviston here are some comments about each of the test setup steps with links to additional info.  For now I've only added them to the `testing-models` page.  If the format and wording of these comments is acceptable I'm happy to add them to each of the other pages where these steps appear.  The entire block of setup steps will look like this:

```javascript
// Globalize ember-qunit. For more info, see http://emberjs.com/guides/testing/unit#toc_unit-testing-setup
emq.globalize();
// Set the resolver for the global context. For more info, see http://emberjs.com/guides/testing/unit#toc_the-resolver
setResolver(App.__container__)
// Place Ember in test mode. For more info, see http://emberjs.com/guides/testing/unit#toc_unit-testing-setup
App.setupForTesting();
```