test: fix test-vm-cached-data to work with old v8

While `let` no longer needs to run in `strict mode` in v8 5.x it throws
in v8 4. This modification will make the test-vm-cached-data work in
older version of node.

Refs: https://github.com/nodejs/node/pull/6280

PR-URL: https://github.com/nodejs/node/pull/6317
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

test/parallel/test-vm-cached-data.js

@@ -14,10 +14,11 @@ function produce(source, count) {
     count = 1;
 
   const out = spawnSync(process.execPath, [ '-e', `
+    'use strict';
     var assert = require('assert');
     var vm = require('vm');
 
-    let data;
+    var data;
     for (var i = 0; i < ${count}; i++) {
       var script = new vm.Script(process.argv[1], {
         produceCachedData: true