test: check that benchmark tests are minimal

Check that benchmark tests are not running longer than necessary by
confirming that they only produce one set of configs to report on per
benchmark file.

PR-URL: https://github.com/nodejs/node/pull/21046


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>