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>