Try to fix flakiness of test standalone/io/raw_secure_server_socket_test

tests/standalone/io/raw_secure_server_socket_test.dart

Index: tests/standalone/io/raw_secure_server_socket_test.dart
diff --git a/tests/standalone/io/raw_secure_server_socket_test.dart b/tests/standalone/io/raw_secure_server_socket_test.dart
index 6618a1d94d1fad7c58afae66aa204a20c3ca7b65..873cfba471ec79c1cba003dc6b045721e5efb546 100644
--- a/tests/standalone/io/raw_secure_server_socket_test.dart
+++ b/tests/standalone/io/raw_secure_server_socket_test.dart
@@ -314,6 +314,7 @@ void testSimpleReadWrite(bool listenSecure,
               // Read first part of TLS handshake from client.
               Expect.isTrue(postponeSecure);
               completer.complete([subscription, buffer]);
+              client.readEventsEnabled = false;
               return;
             }
             data.setRange(bytesRead, bytesRead + buffer.length, buffer);