1. Update fixtures to use a consistent prefix, so we can tell which resources are created by python-bigquery (and possibly disambiguate between samples and system tests). Also include a timestamp in the prefix.
2. List all resources in the project, filter by our prefix, delete those resources that were created >= 24 hours ago.

This will prevent resource leaks from getting too bad. Teardown doesn't always get a chance to run because of intermittent Kokoro errors and timeouts.