Add integration tests for storage

[nijel]

python-social-auth/social-core#986 introduced breaking change in (OpenID) storage API and it went unnoticed into the release. I'm now reverting it in python-social-auth/social-core#1020.

If social-app-django would have integartion tests for these, the error would be noticed earlier.

Exception stack trace:

NotImplementedError: Implement in subclass
  File "django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/cache.py", line 80, in _view_wrapper
    response = view_func(request, *args, **kwargs)
  File "django/views/decorators/http.py", line 64, in inner
    return func(request, *args, **kwargs)
  File "weblate/accounts/views.py", line 1330, in social_auth
    return do_auth(request.backend, redirect_name=REDIRECT_FIELD_NAME)
  File "social_core/actions.py", line 35, in do_auth
    return backend.start()
  File "social_core/backends/base.py", line 33, in start
    if self.uses_redirect():
  File "social_core/backends/open_id.py", line 244, in uses_redirect
    return self.openid_request().shouldSendRedirect()
  File "social_core/backends/open_id.py", line 249, in openid_request
    return self.consumer().begin(url_add_parameters(self.openid_url(), params))
  File "openid/consumer/consumer.py", line 360, in begin
    return self.beginWithoutDiscovery(service, anonymous)
  File "openid/consumer/consumer.py", line 383, in beginWithoutDiscovery
    auth_req = self.consumer.begin(service)
  File "openid/consumer/consumer.py", line 611, in begin
    assoc = self._getAssociation(service_endpoint)
  File "openid/consumer/consumer.py", line 1179, in _getAssociation
    assoc = self.store.getAssociation(endpoint.server_url)
  File "social_core/store.py", line 39, in getAssociation
    for assoc_id, association in self.assoc.oids(server_url, handle):
  File "social_core/storage.py", line 229, in oids
    for assoc in cls.get_association(**kwargs)
  File "social_core/storage.py", line 256, in get_association
    raise NotImplementedError("Implement in subclass")