test: make test-http-expect-continue more strict

In test-http-expect-continue, verify that the request listener is not
called.

PR-URL: https://github.com/nodejs/node/pull/19669


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>