test: fix `http-upgrade-agent` flakiness

It's not guaranteed that the socket data is received in the same chunk
as the upgrade response. Listen for the `data` event to make sure all
the data is received.

PR-URL: https://github.com/nodejs/node/pull/4520


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>