test: use common.PORT instead of an extraneous variable

This test is not parallelized and so we can use the test commons PORT
variable.

Refs: https://github.com/nodejs/node/pull/27565#discussion_r281000162

PR-URL: https://github.com/nodejs/node/pull/27565
Fixes: https://github.com/nodejs/node/issues/27341


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>