test: fix flaky test-net-write-after-close

Replace 250ms timer with event-based logic to make test robust.

PR-URL: nodejs#14361
Fixes: nodejs#13597
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott

test/parallel/test-net-write-after-close.js

@@ -21,24 +21,31 @@
 
 'use strict';
 const common = require('../common');
+
 const net = require('net');
 
+let serverSocket;
+
 const server = net.createServer(common.mustCall(function(socket) {
+  serverSocket = socket;
+
   socket.resume();
 
   socket.on('error', common.mustCall(function(error) {
-    console.error('got error, closing server', error);
+    console.error('received error as expected, closing server', error);
     server.close();
   }));
-
-  setTimeout(common.mustCall(function() {
-    console.error('about to try to write');
-    socket.write('test', common.mustCall());
-  }), 250);
 }));
 
 server.listen(0, function() {
   const client = net.connect(this.address().port, function() {
+    // cliend.end() will close both the readable and writable side
+    // of the duplex because allowHalfOpen defaults to false.
+    // Then 'end' will be emitted when it receives a FIN packet from
+    // the other side.
+    client.on('end', common.mustCall(() => {
+      serverSocket.write('test', common.mustCall());
+    }));
     client.end();
   });
 });