test: modernize JS and tighten equality checking

Changed var --> const and let.
Changed assert.equal !== --> assert.notStrictEqual
Correctly aligned argument

PR-URL: https://github.com/nodejs/node/pull/8580
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

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

@@ -1,21 +1,21 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var path = require('path');
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
 
-var spawn = require('child_process').spawn;
-var childPath = path.join(common.fixturesDir,
-                          'parent-process-nonpersistent.js');
-var persistentPid = -1;
+const spawn = require('child_process').spawn;
+const childPath = path.join(common.fixturesDir,
+                            'parent-process-nonpersistent.js');
+let persistentPid = -1;
 
-var child = spawn(process.execPath, [ childPath ]);
+const child = spawn(process.execPath, [ childPath ]);
 
 child.stdout.on('data', function(data) {
   persistentPid = parseInt(data, 10);
 });
 
 process.on('exit', function() {
-  assert(persistentPid !== -1);
+  assert.notStrictEqual(persistentPid, -1);
   assert.throws(function() {
     process.kill(child.pid);
   });