test: fix flaky test-cluster-send-handle-twice

Use `common.mustCall()` to make sure connection callback runs exactly
once.

Use `connect` event instead of `setTimeout` to avoid test failing if
timer runs before client is connected.

Remove `cluster.worker.disconnect()` after `assert.fail()`. It is
unreachable code that is unnecessary.

PR-URL: nodejs#19700
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-cluster-send-handle-twice.js

@@ -40,21 +40,20 @@ if (cluster.isMaster) {
     }));
   }
 } else {
-  const server = net.createServer(function(socket) {
+  const server = net.createServer(common.mustCall((socket) => {
     process.send('send-handle-1', socket);
     process.send('send-handle-2', socket);
-  });
+  }));
 
   server.listen(0, function() {
     const client = net.connect({
       host: 'localhost',
       port: server.address().port
     });
     client.on('close', common.mustCall(() => { cluster.worker.disconnect(); }));
-    setTimeout(function() { client.end(); }, 50);
+    client.on('connect', () => { client.end(); });
   }).on('error', function(e) {
     console.error(e);
     assert.fail('server.listen failed');
-    cluster.worker.disconnect();
   });
 }