Fix flaky EndToEndTest.ClientAddressSpoofedForSomePeriod as client connection self address does not equal TestLoopback(1) in IPv4 build.

Tested by: blaze test //third_party/quic/core/http:end_to_end_test --test_sharding_strategy=disabled --test_filter=*EndToEndTest.ClientAddressSpoofedForSomePeriod*RFCv1_B2ON --modify_execution_info='TestRunner=+requires-net:ipv4,TestRunner=+requires-net:loopback' --runs_per_test=100

PiperOrigin-RevId: 424879114

quic/core/http/end_to_end_test.cc

diff --git a/quic/core/http/end_to_end_test.cc b/quic/core/http/end_to_end_test.cc
index 12372ed..aa1ef09 100644
--- a/quic/core/http/end_to_end_test.cc
+++ b/quic/core/http/end_to_end_test.cc
@@ -3246,7 +3246,8 @@
   ASSERT_TRUE(QuicConnectionPeer::HasUnusedPeerIssuedConnectionId(
       GetClientConnection()));
 
-  QuicIpAddress real_host = TestLoopback(1);
+  QuicIpAddress real_host =
+      client_->client()->session()->connection()->self_address().host();
   ASSERT_TRUE(client_->MigrateSocket(real_host));
   SendSynchronousFooRequestAndCheckResponse();
   EXPECT_EQ(