benchmark: improve compare output

The current output uses JSON.stringify to escape the config values.
This switches to util.inspect to have a better readable output.

PR-URL: https://github.com/nodejs/node/pull/18597


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>