test: improve coverage for timer promises schedular

PR-URL: #53370
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-timers-promises-scheduler.js

@@ -7,6 +7,7 @@ const { setTimeout } = require('timers');
 const {
   strictEqual,
   rejects,
+  throws,
 } = require('assert');
 
 async function testYield() {
@@ -48,3 +49,7 @@ async function testCancelableWait2() {
 }
 
 testCancelableWait2().then(common.mustCall());
+
+throws(() => new scheduler.constructor(), {
+  code: 'ERR_ILLEGAL_CONSTRUCTOR',
+});