[BUGFIX beta] Fix issue with Ember.Test.unregisterHelper.

* `Ember.Test.unregisterHelper` will currently error if the helper being removed is found within `originalMethods`. The issue is that `this.helperContainer` is not available from `Ember.Test` (it is in `Ember.Application`).
* `orignalMethods` has been moved into a private property of the `Ember.Application` instance (so that we can properly restore the original value).
* Added a few tests for the behavior of `registerHelper` and `unregisterHelper`.

Fixes issue reported by @sly7-7 in IRC.