test: add required settings to test-benchmark-buffer

A benchmark was added but the appropriate settings were not added to
test-benchmark-buffer. These two additions make sure that the new
benchmark file only runs a single benchmark during testing.

PR-URL: https://github.com/nodejs/node/pull/29163


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>