test: dynamic port in cluster worker dgram

Remove common.PORT from test-cluster-dgram-1 and
test-cluster-dgram-2, in order to eliminate the
possibility of port collision.

PR-URL: https://github.com/nodejs/node/pull/12487
Ref: https://github.com/nodejs/node/issues/12376


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>