test: refactor test-stdin-script-child

- var -> const where possible
- assert.equal -> assert.strictEqual
- passed the setTimeout function a second parameter for readability
- used assert.strictEqual for assert(!c) as it is expected to be 0 and
  not some other value

PR-URL: https://github.com/nodejs/node/pull/10321
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

test/parallel/test-stdin-script-child.js

@@ -1,14 +1,14 @@
 'use strict';
-require('../common');
-var assert = require('assert');
+const common = require('../common');
+const assert = require('assert');
 
-var spawn = require('child_process').spawn;
-var child = spawn(process.execPath, [], {
+const spawn = require('child_process').spawn;
+const child = spawn(process.execPath, [], {
   env: Object.assign(process.env, {
     NODE_DEBUG: process.argv[2]
   })
 });
-var wanted = child.pid + '\n';
+const wanted = child.pid + '\n';
 var found = '';
 
 child.stdout.setEncoding('utf8');
@@ -21,12 +21,12 @@ child.stderr.on('data', function(c) {
   console.error('> ' + c.trim().split(/\n/).join('\n> '));
 });
 
-child.on('close', function(c) {
-  assert(!c);
-  assert.equal(found, wanted);
+child.on('close', common.mustCall(function(c) {
+  assert.strictEqual(c, 0);
+  assert.strictEqual(found, wanted);
   console.log('ok');
-});
+}));
 
 setTimeout(function() {
   child.stdin.end('console.log(process.pid)');
-});
+}, 1);