test: refactored test-http-response-splitting to use countdown

PR-URL: https://github.com/nodejs/node/pull/17348


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>