Can not override auth config options from tests

When writing auth tests, using the `@helpers.change_config()` decorator on an auth option has no effect. The reason is that auth functions call [`new_authz.py.check_config_permission`](https://github.com/ckan/ckan/blob/04e6ac4c0363bb2923c27ef6bad398b3f73ac819/ckan/new_authz.py#L380) which caches the config and default values the first time is run.

I think is worth not caching the values and allowing them to be overriden from tests than the performance gains.