The minimum value currently allowed for the `HeartbeatPeriod` config option is 30 minutes. It is useful in testing Tor networks (`TestingTorNetwork 1`) to log heartbeat information more often than every 30 minutes. This is a request to change Tor so that it does not enforce a minimum value for the `HeartbeatPeriod` in the case that `TestingTorNetwork` is set to 1. Patch attached.