Allow HeartbeatPeriod of less than 30 minutes in testing Tor networks

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.

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