test, http: add regression test for keepalive 'end' event

This test covers a regression where 'end' was not emitted in the case
of keepalive requests without parsing the full body.

PR-URL: https://github.com/nodejs/node/pull/29263


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>