Miscellaneous test changes for IETF QUIC mTLS support:
- Make gfe2::test::SetClientCert work for IETF QUIC client.
- Make gfe2::test::SimpleQuicClient::connection_error const.
- Change ClientCertRequestsTest.TestClientCertsHeadersStripped to verify the value of GFE_SSL_CERTIFICATE is replaced by GFE, rather than verifying the value is of a specific value.

Split from cl/402574728.

PiperOrigin-RevId: 404540354
diff --git a/quic/test_tools/quic_test_client.cc b/quic/test_tools/quic_test_client.cc
index 6b1f831..02d890f 100644
--- a/quic/test_tools/quic_test_client.cc
+++ b/quic/test_tools/quic_test_client.cc
@@ -618,7 +618,7 @@
   return latest_created_stream_;
 }
 
-QuicErrorCode QuicTestClient::connection_error() {
+QuicErrorCode QuicTestClient::connection_error() const {
   return client()->connection_error();
 }
 
diff --git a/quic/test_tools/quic_test_client.h b/quic/test_tools/quic_test_client.h
index 66ebb5b..db97e02 100644
--- a/quic/test_tools/quic_test_client.h
+++ b/quic/test_tools/quic_test_client.h
@@ -268,7 +268,7 @@
       QuicReferenceCountedPointer<QuicAckListenerInterface> ack_listener);
 
   QuicRstStreamErrorCode stream_error() { return stream_error_; }
-  QuicErrorCode connection_error();
+  QuicErrorCode connection_error() const;
 
   MockableQuicClient* client() { return client_.get(); }
   const MockableQuicClient* client() const { return client_.get(); }