Use UrlResetMixin to test GroupConfiguration feature

Previously GroupConfiguration were skipped unless flag was set in FEATURES, to avoid troubles with urlconf changes. Fortunately, we have a mixin for dealing with such troubles, and therefore can make those tests run unconditionally.

Also, this change fixes two failing tests which were not updated to reflect change in https://github.com/edx/edx-platform/commit/e8ae3d1b4a652ea69ab5976da4d6bef4e9a178f7#diff-0

review: @cahrens @auraz @olmar