test: deflake test-http2-buffersize

This is basically a revert of
https://github.com/nodejs/node/commit/c452632d3421d6baf26a61e14c21c922bd18175e.

The problem with that commit is that the promises returned by
`once(server, 'stream')` are all resolved with the same stream when the
first `'stream'` event is emitted.

Refs: https://github.com/nodejs/node/pull/39525#issuecomment-889080913

PR-URL: https://github.com/nodejs/node/pull/39591


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>