Need automated test coverage for hostdb's "serve_stale_for"

config option: https://docs.trafficserver.apache.org/en/latest/admin-guide/files/records.config.en.html?highlight=records.config#proxy-config-hostdb-serve-stale-for

This feature allows hostdb to continue serving stale records until a new one can be acquired. As of today we have no real tests around this, so to do that we'll need to first make ATS support port numbers in resolv.conf-- then we can easily wire up some tsqa tests to test the various failure modes.