test: update test/parallel/test-child-process-send-utf8.js

change var to const
use arrow function in callback
use strict equal

PR-URL: https://github.com/nodejs/node/pull/8594
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-child-process-send-utf8.js

@@ -1,14 +1,14 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var fork = require('child_process').fork;
+const common = require('../common');
+const assert = require('assert');
+const fork = require('child_process').fork;
 
-var expected = Array(1e5).join('ßßßß');
+const expected = Array(1e5).join('ßßßß');
 if (process.argv[2] === 'child') {
   process.send(expected);
 } else {
-  var child = fork(process.argv[1], ['child']);
-  child.on('message', common.mustCall(function(actual) {
-    assert.equal(actual, expected);
+  const child = fork(process.argv[1], ['child']);
+  child.on('message', common.mustCall((actual) => {
+    assert.strictEqual(actual, expected);
   }));
 }