Automatically test the PTs of bridges

When a new bridge is set up, our directory authority is testing its OR port and assigns it the Running flag if the OR port is reachable. Nothing however is testing a bridge's PT port(s). This resulted in several bridges having an unreachable obfs4 port, e.g., because the operator failed to whitelist the obfs4 port in their firewall. Let's fix this by testing a bridge's pluggable transport and alerting the operator if the PT is unreachable.

Obfs4proxy has client implementations for most of our currently-deployed PTs, so we could start by writing some glue code that takes as input a bridge line and makes obfs4proxy (and tor) connect to the given bridge.

Another question is where we should do the testing from. Our bridge authority and BridgeDB are the obvious candidates. Our bridge authority currently tests bridges' OR ports but we may not want it to also test PTs.

Finally, how should we let bridge operators know if their PTs are unreachable? We may want to send them an email (if they have contact information in their descriptor), and/or make their tor log a warning.