Fixes an Envoy continuous integration build error:

external/com_github_google_quiche/quiche/http2/adapter/test_utils.cc: In member function ‘virtual bool http2::adapter::test::TestDataFrameSource::Send(absl::string_view, size_t)’:
external/com_github_google_quiche/quiche/http2/adapter/test_utils.cc:64:46: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
   64 |   } else if (static_cast<const size_t>(result) < concatenated.size()) {
      |

PiperOrigin-RevId: 449212715
diff --git a/quiche/http2/adapter/test_utils.cc b/quiche/http2/adapter/test_utils.cc
index 5d6d427..0d08d27 100644
--- a/quiche/http2/adapter/test_utils.cc
+++ b/quiche/http2/adapter/test_utils.cc
@@ -61,7 +61,7 @@
   } else if (result == 0) {
     // Write blocked.
     return false;
-  } else if (static_cast<const size_t>(result) < concatenated.size()) {
+  } else if (static_cast<size_t>(result) < concatenated.size()) {
     // Probably need to handle this better within this test class.
     QUICHE_LOG(DFATAL)
         << "DATA frame not fully flushed. Connection will be corrupt!";