test: remove hidden use of common.PORT in parallel tests

common.hasMultiLocalhost() uses common.PORT under the hood. This is
problematic in parallel tests because another test using port 0 to
get an arbitrary open port may end up getting common.PORT before the
test using common.PORT gets it.

Therefore, change common.PORT to port 0 in common.hasMultiLocalhost().

PR-URL: https://github.com/nodejs/node/pull/17466


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>