test: update test-cluster-disconnect-unshared-udp

Changed var to const

PR-URL: https://github.com/nodejs/node/pull/8599


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>