test: remove duplicate required module

`common` is required twice in test-setproctitle.js. Remove one of the
instances.

Other refactoring:

* var -> const and let
* assert.equal -> assert.strictEqual
* assert.notEqual -> assert.notStrickEqual
* string concatenation -> template string
* use of assert.ifError() instead of asserting error is null

PR-URL: https://github.com/nodejs/node/pull/9169
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-setproctitle.js

@@ -8,27 +8,27 @@ if ('linux freebsd darwin'.indexOf(process.platform) === -1) {
   return;
 }
 
-var assert = require('assert');
+const assert = require('assert');
-var exec = require('child_process').exec;
+const exec = require('child_process').exec;
-var path = require('path');
+const path = require('path');
 
 // The title shouldn't be too long; libuv's uv_set_process_title() out of
 // security considerations no longer overwrites envp, only argv, so the
 // maximum title length is possibly quite short.
-var title = 'testme';
+let title = 'testme';
 
-assert.notEqual(process.title, title);
+assert.notStrictEqual(process.title, title);
 process.title = title;
-assert.equal(process.title, title);
+assert.strictEqual(process.title, title);
 
-exec('ps -p ' + process.pid + ' -o args=', function(error, stdout, stderr) {
+exec(`ps -p ${process.pid} -o args=`, function callback(error, stdout, stderr) {
-  assert.equal(error, null);
+  assert.ifError(error);
-  assert.equal(stderr, '');
+  assert.strictEqual(stderr, '');
 
   // freebsd always add ' (procname)' to the process title
   if (process.platform === 'freebsd')
     title += ` (${path.basename(process.execPath)})`;
 
   // omitting trailing whitespace and \n
-  assert.equal(stdout.replace(/\s+$/, ''), title);
+  assert.strictEqual(stdout.replace(/\s+$/, ''), title);
 });