test: make test-http2-buffersize more correct

Previously, this code could have closed the server before the
connection was actually received by the server, as the `'close'`
event on the client side can be emitted before the connection
is established.

The following commit exacerbates this problem, so fix the test first.

PR-URL: https://github.com/nodejs/node/pull/31502


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>