benchmark: swap var for let in benchmarks

In benchmark directory this changes for loops
using var to let when it applies for consistency

benchmark/module/module-loader-deep.js

@@ -19,7 +19,7 @@ function main({ ext, cache, files }) {
     `${benchmarkDirectory}/a.js`,
     'module.exports = {};'
   );
-  for (var i = 0; i <= files; i++) {
+  for (let i = 0; i <= files; i++) {
     fs.mkdirSync(`${benchmarkDirectory}/${i}`);
     fs.writeFileSync(
       `${benchmarkDirectory}/${i}/package.json`,

benchmark/module/module-loader.js

@@ -21,7 +21,7 @@ const bench = common.createBenchmark(main, {
 function main({ n, name, cache, files, dir }) {
   tmpdir.refresh();
   fs.mkdirSync(benchmarkDirectory);
-  for (var i = 0; i <= files; i++) {
+  for (let i = 0; i <= files; i++) {
     fs.mkdirSync(`${benchmarkDirectory}${i}`);
     fs.writeFileSync(
       `${benchmarkDirectory}${i}/package.json`,

benchmark/string_decoder/string-decoder-create.js

@@ -11,7 +11,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ encoding, n }) {
   bench.start();
-  for (var i = 0; i < n; ++i) {
+  for (let i = 0; i < n; ++i) {
     const sd = new StringDecoder(encoding);
     !!sd.encoding;
   }

benchmark/tls/convertprotocols.js

@@ -16,7 +16,7 @@ function main({ n }) {
     m = {};
   }
   bench.start();
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     tls.convertALPNProtocols(input, m);
   bench.end(n);
 }

benchmark/tls/tls-connect.js

@@ -31,7 +31,7 @@ function main(conf) {
 function onListening() {
   setTimeout(done, dur * 1000);
   bench.start();
-  for (var i = 0; i < concurrency; i++)
+  for (let i = 0; i < concurrency; i++)
     makeConnection();
 }
 

benchmark/v8/get-stats.js

@@ -13,7 +13,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ method, n }) {
   bench.start();
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     v8[method]();
   bench.end(n);
 }

benchmark/worker/echo.js

@@ -32,7 +32,7 @@ function main({ n, workers, sendsPerBroadcast: sends, payload: payloadType }) {
 
   const workerObjs = [];
 
-  for (var i = 0; i < workers; ++i) {
+  for (let i = 0; i < workers; ++i) {
     const worker = new Worker(workerPath);
     workerObjs.push(worker);
     worker.on('online', onOnline);