test: improve coverage of lib/_http_outgoing.js

Refs: https://coverage.nodejs.org/coverage-36bb31be5f0b85a0/lib/_http_outgoing.js.html#L351
Refs: https://coverage.nodejs.org/coverage-36bb31be5f0b85a0/lib/_http_outgoing.js.html#L609

PR-URL: https://github.com/nodejs/node/pull/38734


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>