test: fix http-agent-destroyed-socket cb not firing

A whole part of the test-http-agent-destroyed-socket test
was not running as the semantics of http events changed
slightly and were no longer triggering the expected event.
Instead listen to the same event on the socket to verify
that the code being tested is still working as expected.

PR-URL: https://github.com/nodejs/node/pull/20006
Fixes: https://github.com/nodejs/node/issues/8613


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>