benchmark: replace more [].join() with ''.repeat()

Refs: https://github.com/nodejs/node/pull/12170

PR-URL: https://github.com/nodejs/node/pull/12317
Reviewed-By: James M Snell <jasnell@gmail.com>

benchmark/child_process/child-process-read.js

@@ -19,7 +19,7 @@ function main(conf) {
   const dur = +conf.dur;
   const len = +conf.len;
 
-  const msg = '"' + Array(len).join('.') + '"';
+  const msg = `"${'.'.repeat(len)}"`;
   const options = { 'stdio': ['ignore', 'pipe', 'ignore'] };
   const child = spawn('yes', [msg], options);
 

benchmark/http/_chunky_http_client.js

@@ -20,7 +20,7 @@ function main(conf) {
   // Chose 7 because 9 showed "Connection error" / "Connection closed"
   // An odd number could result in a better length dispersion.
   for (var i = 7; i <= 7 * 7 * 7; i *= 7)
-    headers.push(Array(i + 1).join('o'));
+    headers.push('o'.repeat(i));
 
   function WriteHTTPHeaders(channel, has_keep_alive, extra_header_count) {
     todo = [];