test: fix flaky test-https-agent-create-connection

Use a different server instance for every test to avoid that they reuse
the same port.

PR-URL: https://github.com/nodejs/node/pull/11649
Fixes: https://github.com/nodejs/node/issues/11644


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>