test: fix flaky test-net-connect-econnrefused

Test is flaky in CI with `common.PORT` but not flaky if a port is
determined from createServer() first.

PR-URL: https://github.com/nodejs/node/pull/34330


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>