test: reduce benchmark cases in test-benchmark-buffer

Specify `buffer=fast` so that we only run that and not that along with
`buffer=slow` in two benchmarks.

PR-URL: https://github.com/nodejs/node/pull/17111


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>