test: add mustCall in timers-unrefed-in-callback

PR-URL: #12594
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

test/parallel/test-timers-unrefed-in-callback.js

@@ -2,8 +2,7 @@
 // Checks that setInterval timers keep running even when they're
 // unrefed within their callback.
 
-require('../common');
-const assert = require('assert');
+const common = require('../common');
 const net = require('net');
 
 let counter1 = 0;
@@ -28,15 +27,15 @@ function Test1() {
   // server only for maintaining event loop
   const server = net.createServer().listen(0);
 
-  const timer1 = setInterval(() => {
+  const timer1 = setInterval(common.mustCall(() => {
     timer1.unref();
     if (counter1++ === 3) {
       clearInterval(timer1);
       server.close(() => {
         Test2();
       });
     }
-  }, 1);
+  }, 4), 1);
 }
 
 
@@ -54,8 +53,4 @@ function Test2() {
   }, 1);
 }
 
-process.on('exit', () => {
-  assert.strictEqual(counter1, 4);
-});
-
 Test1();