test: improve test-child-process-stdout-flush

Changed vars to const / let, functions to arrow functions and a
mustCall where appropriate.

PR-URL: https://github.com/nodejs/node/pull/8581
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>

test/parallel/test-child-process-stdout-flush.js

@@ -1,29 +1,26 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var path = require('path');
-var spawn = require('child_process').spawn;
-var sub = path.join(common.fixturesDir, 'print-chars.js');
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const spawn = require('child_process').spawn;
+const sub = path.join(common.fixturesDir, 'print-chars.js');
 
-var n = 500000;
+const n = 500000;
 
-var child = spawn(process.argv[0], [sub, n]);
+const child = spawn(process.argv[0], [sub, n]);
 
-var count = 0;
+let count = 0;
 
 child.stderr.setEncoding('utf8');
-child.stderr.on('data', function(data) {
-  console.log('parent stderr: ' + data);
-  assert.ok(false);
-});
+child.stderr.on('data', common.fail);
 
 child.stdout.setEncoding('utf8');
-child.stdout.on('data', function(data) {
+child.stdout.on('data', (data) => {
   count += data.length;
-  console.log(count);
 });
 
-child.on('close', function(data) {
-  assert.equal(n, count);
-  console.log('okay');
-});
+child.on('close', common.mustCall((code, signal) => {
+  assert.strictEqual(code, 0);
+  assert.strictEqual(signal, null);
+  assert.strictEqual(n, count);
+}));