ospf6d: fix topotest

ospf6d: fix topotest

The routes in the test_nssa_range() are E2

  "routes":{
    "2001:db8:1000::2/128":{
      "destinationType":"Network",
      "installedTimeSince":"00:06:29",
      "changedTimeSince":"00:06:29",
      "numberOfLock":2,
      "flags":"BA--",
      "associatedArea":"0.0.0.2",
      "pathType":"External-2",
      "lsOriginRoutePathType":"NSSA",
      "lsId":"0.0.0.3",
      "lsAdvertisingRouter":"10.254.254.4",
      "options":"--|-|-|--|-|--",
      "routerBits":"--------",
      "prefixOptions":"--|P|--|--|--",
      "metricType":2,
      "metricCost":10,
      "metricCostE2":20,
      "pathsCount":1,
      "nextHopCount":1,
      "nextHops":[
        {
          "nextHop":"::",
          "interfaceName":"r2-eth2"
        }
      ]
    }

This PR fixes the key from `metricCost` to `metricCostE2`

Signed-off-by: ckishimo <carles.kishimoto@gmail.com>