Add unique serial numbers to sample X509 certificates. Remove the "sendClientCertificate" parameter…

tests/standalone/io/secure_server_client_certificate_test.dart

Index: tests/standalone/io/secure_server_client_certificate_test.dart
diff --git a/tests/standalone/io/secure_server_client_certificate_test.dart b/tests/standalone/io/secure_server_client_certificate_test.dart
index 81f0995a6af14854051f86e287cebd9eae602b20..c104dbaf545f197ae818ba0c0476d369ccc9b9ae 100644
--- a/tests/standalone/io/secure_server_client_certificate_test.dart
+++ b/tests/standalone/io/secure_server_client_certificate_test.dart
@@ -32,8 +32,8 @@ Future testClientCertificate({bool required, bool sendCert}) async {
   var server = await SecureServerSocket.bind(HOST, 0, serverContext,
       requestClientCertificate: true, requireClientCertificate: required);
   var clientContext = sendCert ? clientCertContext : clientNoCertContext;
-  var clientEndFuture = SecureSocket.connect(HOST, server.port,
-      context: clientContext, sendClientCertificate: true);
+  var clientEndFuture =
+      SecureSocket.connect(HOST, server.port, context: clientContext);
   if (required && !sendCert) {
     try {
       await server.first;