test: pass through stderr in benchmark tests

This helps a lot with debugging failing benchmark tests,
which would otherwise just print an assertion for the
exit code (something like `+1 -0`, which yields almost no
information about a failure).

PR-URL: https://github.com/nodejs/node/pull/21860


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>