Let QuicTestClient wait for all pending ACKs to be flushed in WaitForResponseMs, such that server can avoid unnecessary retransmissions in tests.

PiperOrigin-RevId: 314721238
Change-Id: I52f77afb82c75715afdf5e78d52ffb678714ed93
diff --git a/quic/core/quic_connection.h b/quic/core/quic_connection.h
index 969da12..e43453a 100644
--- a/quic/core/quic_connection.h
+++ b/quic/core/quic_connection.h
@@ -982,6 +982,9 @@
   void OnTransportParametersReceived(
       const TransportParameters& transport_parameters) const;
 
+  // Returns true if ack_alarm_ is set.
+  bool HasPendingAcks() const;
+
   size_t anti_amplification_factor() const {
     return anti_amplification_factor_;
   }