benchmark,test: remove output from readable-async-iterator benchmark

Extra output makes test-benchmark-streams fail. Change console.log() to
console.assert().

Fixes: https://github.com/nodejs/node/issues/34409

PR-URL: https://github.com/nodejs/node/pull/34411


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>