test: use mustCall in place of countdown in timers test

Use common.mustCall() in place of countdown in
test-timers-immediate-unref.

PR-URL: #32416
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-timers-immediate-unref.js

@@ -1,8 +1,6 @@
 'use strict';
 
 const common = require('../common');
-const Countdown = require('../common/countdown');
-
 const assert = require('assert');
 
 const immediate = setImmediate(() => {});
@@ -16,12 +14,10 @@ clearImmediate(immediate);
 setImmediate(common.mustCall(firstStep)).ref().unref().unref().ref();
 
 function firstStep() {
-  const countdown =
-    new Countdown(2, common.mustCall(() => setImmediate(secondStep)));
   // Unrefed setImmediate executes if it was unrefed but something else keeps
   // the loop open
-  setImmediate(() => countdown.dec()).unref();
-  setTimeout(() => countdown.dec(), 50);
+  setImmediate(common.mustCall()).unref();
+  setTimeout(common.mustCall(() => { setImmediate(secondStep); }), 0);
 }
 
 function secondStep() {