test: add mustCall to net-connect-buffer test

PR-URL: #27446
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-net-connect-buffer.js

@@ -33,11 +33,11 @@ const tcp = net.Server(common.mustCall((s) => {
     buf += d;
   });
 
-  s.on('end', function() {
+  s.on('end', common.mustCall(function() {
     console.error('SERVER: end', buf);
     assert.strictEqual(buf, "L'État, c'est moi");
     s.end();
-  });
+  }));
 }));
 
 tcp.listen(0, common.mustCall(function() {