Confirm that port forwarding works correctly

We ran into a configuration error with op-hk5 where we didn't set up port forwarding properly, and it took us two weeks to figure that out. @asn experienced a similar problem with not setting up port forwarding correctly. Let's see if we can make this a little more usable.

How about OnionPerf performs a self test by connecting with its TGen client to its own TGen server directly? This could happen just once at startup, and only if listen port and connect port are different. Or does that not work, because port forwarding works differently for connecting from localhost as compared to connecting from an exit node? Are there alternatives?