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

benchmark/child_process/child-process-exec-stdout.js

@@ -20,8 +20,7 @@ function main(conf) {
   const msg = `"${'.'.repeat(len)}"`;
   msg.match(/./);
   const options = {'stdio': ['ignore', 'pipe', 'ignore']};
-  // NOTE: Command below assumes bash shell.
+  const child = exec(`yes ${msg}`, options);
-  const child = exec(`while\n  echo ${msg}\ndo :; done\n`, options);
 
   var bytes = 0;
   child.stdout.on('data', function(msg) {