Add a simple end to end test that ensures that enabling the spin bit doesn't break Quiche.

Useful because the unit tests for the spin bit itself just verifies the spin and spin enablement logic; this adds additional assurance that the flag doesn't break for other unknown reasons.

Protected by FLAGS_quic_reloadable_flag_quic_enable_spin_bit.

PiperOrigin-RevId: 889666317

quiche/quic/core/http/end_to_end_test.cc

diff --git a/quiche/quic/core/http/end_to_end_test.cc b/quiche/quic/core/http/end_to_end_test.cc
index 9f49142..9035a26 100644
--- a/quiche/quic/core/http/end_to_end_test.cc
+++ b/quiche/quic/core/http/end_to_end_test.cc
@@ -8803,6 +8803,14 @@
   client_->Disconnect();
 }
 
+TEST_P(EndToEndTest, SpinBitEnableSafety) {
+  SetQuicReloadableFlag(quic_enable_spin_bit, true);
+
+  ASSERT_TRUE(Initialize());
+
+  SendSynchronousFooRequestAndCheckResponse();
+}
+
 }  // namespace
 }  // namespace test
 }  // namespace quic