test: dynamic port in dgram tests

Removed common.PORT from test-dgram-close-in-listening,
test-dgram-close-is-not-callback, test-dgram-close,
test-dgram-exclusive-implicit-bind and test-dgram-oob-buffer
in order to eliminate the possibility of port collision.

Refs: https://github.com/nodejs/node/issues/12376
PR-URL: https://github.com/nodejs/node/pull/12623


Reviewed-By: Anna Henningsen <anna@addaleax.net>