iperf may be run in simultaneous downlink/uplink mode using --dualtest option

In such case client side starts iperf server process as well and server side establishes second client socket.

If there is communication problem (wrong routing, blocking proxy, etc) only one socket is established on both sides. Command seems to run ok but final report for the other direction will be missing.

We should check (at command startup) if both sockets have been established for --dualtest and if not we quickly raise exception.