test: add mustCall in test-timers-clearImmediate

PR-URL: nodejs#12598
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-timers-clearImmediate.js

@@ -1,18 +1,13 @@
 'use strict';
-require('../common');
-const assert = require('assert');
+const common = require('../common');
 
 const N = 3;
-let count = 0;
+
 function next() {
-  const immediate = setImmediate(function() {
-    clearImmediate(immediate);
-    ++count;
-  });
+  const fn = common.mustCall(() => clearImmediate(immediate));
+  const immediate = setImmediate(fn);
 }
-for (let i = 0; i < N; ++i)
-  next();
 
-process.on('exit', () => {
-  assert.strictEqual(count, N, `Expected ${N} immediate callback executions`);
-});
+for (let i = 0; i < N; i++) {
+  next();
+}