test: wrap callback in common.mustCall

PR-URL: #17173
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-cluster-message.js

@@ -1,5 +1,5 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const cluster = require('cluster');
 const net = require('net');
@@ -111,9 +111,9 @@ if (cluster.isWorker) {
       worker.kill();
     });
 
-    worker.on('exit', function() {
+    worker.on('exit', common.mustCall(function() {
       process.exit(0);
-    });
+    }));
   });
 
   process.once('exit', function() {