Make dispatcher VN client CID test work for all flag values

We noticed that QuicDispatcherTest.StatelessVersionNegotiationWithClientConnectionId fails on Chromium because quic_allow_variable_length_connection_id_for_negotiation is false. This uncovered that this test relies on that flag being true. To avoid issues in the future if we have to flip this flag back, this CL changes this test to set the flag to true.

gfe-relnote: n/a, test-only
PiperOrigin-RevId: 252026292
Change-Id: I63c928e3b174cfff0c71e9fc4a971ff6d2a7ffd0
diff --git a/quic/core/quic_dispatcher_test.cc b/quic/core/quic_dispatcher_test.cc
index 305d359..9991695 100644
--- a/quic/core/quic_dispatcher_test.cc
+++ b/quic/core/quic_dispatcher_test.cc
@@ -517,6 +517,8 @@
 
 TEST_F(QuicDispatcherTest, StatelessVersionNegotiationWithClientConnectionId) {
   SetQuicRestartFlag(quic_do_not_override_connection_id, true);
+  SetQuicRestartFlag(quic_allow_variable_length_connection_id_for_negotiation,
+                     true);
   CreateTimeWaitListManager();
   QuicSocketAddress client_address(QuicIpAddress::Loopback4(), 1);