test: dynamic port in parallel regress tests

Removed common.PORT from test-regress-GH-5051 and
test-regress-GH-5727 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/12639


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>