Fix open-source build

cl/477725360 broke the open-source build, for example in Chromium

net/third_party/quiche/src/quiche/quic/core/quic_dispatcher_test.cc:2298:30: error: no member named 'UnconditionallyAddNewConnectionIdForTest' in 'quic::test::(anonymous namespace)::MockServerConnection'
    mock_server_connection1->UnconditionallyAddNewConnectionIdForTest(
    ~~~~~~~~~~~~~~~~~~~~~~~  ^

PiperOrigin-RevId: 478466288
diff --git a/quiche/quic/core/quic_dispatcher_test.cc b/quiche/quic/core/quic_dispatcher_test.cc
index 0382fdb..0447586 100644
--- a/quiche/quic/core/quic_dispatcher_test.cc
+++ b/quiche/quic/core/quic_dispatcher_test.cc
@@ -177,6 +177,11 @@
     active_connection_ids_.push_back(id);
   }
 
+  void UnconditionallyAddNewConnectionIdForTest(QuicConnectionId id) {
+    dispatcher_->TryAddNewConnectionId(active_connection_ids_.back(), id);
+    active_connection_ids_.push_back(id);
+  }
+
   void RetireConnectionId(QuicConnectionId id) {
     auto it = std::find(active_connection_ids_.begin(),
                         active_connection_ids_.end(), id);