test: cleanup test-child-process-stdio.js

- refactor vars to const and let

issue: nodejs/code-and-learn#56(comment)

modified file: test/parallel/test-child-process-stdio.js

refactor variable declaration "var common" to "const common"
refactor variable declaration "var assert" to "const assert"
refactor variable declaration "var options" to "let options"
refactor variable declaration "var child" to "let child"

Author: matzavinos

test/parallel/test-child-process-stdio.js

@@ -1,9 +1,9 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
+const assert = require('assert');
 
-var options = {stdio: ['pipe']};
-var child = common.spawnPwd(options);
+let options = {stdio: ['pipe']};
+let child = common.spawnPwd(options);
 
 assert.notEqual(child.stdout, null);
 assert.notEqual(child.stderr, null);