test: fix flaky timers-block-eventloop test

Due to extensive reliance on timings and the fs module, this test
is currently inherently flaky. Refactor it to simply use setImmediate
and only one busy loop.

PR-URL: #18567
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: apapirovski

test/sequential/test-timers-block-eventloop.js

@@ -1,24 +1,18 @@
 'use strict';
 
 const common = require('../common');
-const fs = require('fs');
-const platformTimeout = common.platformTimeout;
+const assert = require('assert');
 
+let called = false;
 const t1 = setInterval(() => {
-  common.busyLoop(platformTimeout(12));
-}, platformTimeout(10));
-
-const t2 = setInterval(() => {
-  common.busyLoop(platformTimeout(15));
-}, platformTimeout(10));
-
-const t3 =
-  setTimeout(common.mustNotCall('eventloop blocked!'), platformTimeout(200));
-
-setTimeout(function() {
-  fs.stat('/dev/nonexistent', () => {
+  assert(!called);
+  called = true;
+  setImmediate(common.mustCall(() => {
     clearInterval(t1);
     clearInterval(t2);
-    clearTimeout(t3);
-  });
-}, platformTimeout(50));
+  }));
+}, 10);
+
+const t2 = setInterval(() => {
+  common.busyLoop(20);
+}, 10);