test: increase coverage for _http_incoming.js

Add coverage for IncomingMessage.prototype.connection setter.

PR-URL: https://github.com/nodejs/node/pull/31093


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>