Write test confirming someone can't fetch all leaderboards

https://github.com/codalab/competitions-v2/blob/749fb95be29964088b8c71c398e78b0a83c13404/src/apps/api/tests/test_leaderboards.py#L26

When hitting /api/leaderboards/ (or whatever the endpoint is) confirm that nothing is returned. We should only return leaderboards filtered by competition.