test: refactor test-fs-watch-stop-async

* use common.mustNotCall() to confirm callback is not invoked
* blank line after common module per test writing guide

PR-URL: nodejs#13661
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-fs-watch-stop-async.js

@@ -1,9 +1,10 @@
 'use strict';
 const common = require('../common');
+
 const assert = require('assert');
 const fs = require('fs');
 
-const watch = fs.watchFile(__filename, common.noop);
+const watch = fs.watchFile(__filename, common.mustNotCall());
 let triggered;
 const listener = common.mustCall(() => {
   triggered = true;