[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`.
Merge pull request #3775 from rjackson/fix_issue_with_unregistering_test_helpers

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