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