Regression tests for ncks --jsn

[czender]

Please add regression tests to verify ncks --jsn option works, e.g.,

zender@aerosol:~$ ncks --jsn -v one ~/nco/data/in.nc
{
  "variables": {
    "one": {
      "type": "float",
      "attributes": {
        "long_name": "one"
      },
      "data": 1.0
    }
  }
}

[czender]

Please update the status of this.