extract load-initializers and use it in test-loader

https://github.com/stefanpenner/ember-app-kit/pull/579 added initalizers autoloading (thanks!)

This PR attempts to extract that into a separate model and adds the same behaviour to test loader so that they're available during tests.

`test-loader.js` seems the right place to do it as it has to happen only once (`App.destroy()` won't remove previously added initializers, nor it should)

Let me know if this makes sense or if I'm just missing a much better way to deal with it.

Thanks!