Assertions should only be used in tests - they can be disabled at
runtime by setting the python flag -O (though I don't believe we use
that flag under normal circumstances).
also clean up test asserts - mock_redis is the redis object, so its
`called` property will always be false, because we never say
`redis_store()`. Rather, we should use the `mock_calls` property to
see all calls to all of its children