benchmark: use 'yes' instead of echo in a loop

This changes child-process-exec-stdout benchmark to use 'yes' instead
of echo in a while loop. This makes this benchmark consistent with
child-process-read which already uses `yes` and allows this benchmark
to be executed on Windows.

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/node/pull/8721