test: use dynamic port in test-cluster-dgram-reuse

Remove common.PORT from test-cluster-dgram-reuse to eliminate
possibility that a dynamic port used in another test will collide with
common.PORT.

PR-URL: https://github.com/nodejs/node/pull/12901
Ref: https://github.com/nodejs/node/issues/12376


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>