[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`.