Deflake quic_generic_session_test

In cl/564691072, I increased timeout from 2 RTTs to 4 RTTs, but I didn't change
the number in the "wait for the last flight to arrive" part, and apparently
that is causing test flakes.

PiperOrigin-RevId: 565059324
diff --git a/quiche/quic/core/quic_generic_session_test.cc b/quiche/quic/core/quic_generic_session_test.cc
index a7bc591..16febf0 100644
--- a/quiche/quic/core/quic_generic_session_test.cc
+++ b/quiche/quic/core/quic_generic_session_test.cc
@@ -383,7 +383,7 @@
       4 * simulator::TestHarness::kServerBandwidth.TransferTime(
               1000 * kMaxOutgoingPacketSize)));
   // Allow extra round-trips for the final flight of datagrams to arrive back.
-  test_harness_.simulator().RunFor(2 * simulator::TestHarness::kRtt);
+  test_harness_.simulator().RunFor(4 * simulator::TestHarness::kRtt);
 
   QuicPacketCount client_lost =
       client_->session()->GetDatagramStats().lost_outgoing;