Use TCPInfo.MinRTT instead of TCPInfo.RTT

[robertodauria]

Show MinRTT instead of RTT in the test summary. Examples:

$ ./ndt7-client -quiet -format=json | jq
{
  "ServerFQDN": "ndt-iupui-mlab1-mil04.mlab-oti.measurement-lab.org",
  "ServerIP": "213.242.77.139",
  "ClientIP": "...",
  "DownloadUUID": "ndt-wnkr5_1589391933_0000000000051384",
  "Download": {
    "Value": 389.681259751377,
    "Unit": "Mbit/s"
  },
  "Upload": {
    "Value": 38.33447624961975,
    "Unit": "Mbit/s"
  },
  "DownloadRetrans": {
    "Value": 0.5296091494852461,
    "Unit": "%"
  },
  "MinRTT": {
    "Value": 22,
    "Unit": "ms"
  }
}

$ ./ndt7-client -quiet
         Server: ndt-iupui-mlab2-mil04.mlab-oti.measurement-lab.org
         Client: ...
        Latency:    22.0 ms
       Download:   362.8 Mbit/s
         Upload:    45.8 Mbit/s
 Retransmission:    0.76 %

Thanks @mattmathis for recommending this.

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This change is 

[stephen-soltesz]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @stephen-soltesz)