Fix gQUIC remote handshake by postpone certificate validation to after GetProof is done.

Also modify standalone_handshaker_end_to_end_test to verify the HTTP response content, instead of response_complete(), which is implemented incorrectly in quic client.

Protected by FLAGS_quic_reloadable_flag_quic_crypto_postpone_cert_validate_for_server.

PiperOrigin-RevId: 360229986
Change-Id: I1d7afa111daec727901c1e6a8c1c95ca4c76e731
diff --git a/quic/test_tools/quic_test_client.cc b/quic/test_tools/quic_test_client.cc
index 4133950..d1c1272 100644
--- a/quic/test_tools/quic_test_client.cc
+++ b/quic/test_tools/quic_test_client.cc
@@ -796,7 +796,8 @@
   closed_stream_states_.insert(std::make_pair(
       id,
       PerStreamState(
-          client_stream->stream_error(), true,
+          // Set response_complete to true iff stream is closed while connected.
+          client_stream->stream_error(), connected(),
           client_stream->headers_decompressed(),
           client_stream->response_headers(),
           client_stream->preliminary_headers(),