test,http: fix http dump test

Make sure the dump test actually verify what is happening and it is
not flaky.

See: https://github.com/nodejs/node/issues/19139
PR-URL: https://github.com/nodejs/node/pull/19823


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