Make sure mock_web_transport ends up in the correct file list

To ensure that all other test-only files end up in the correct file list, move
it into its own directory.

PiperOrigin-RevId: 500563814
diff --git a/build/source_list.gni b/build/source_list.gni
index 99ebb16..5779607 100644
--- a/build/source_list.gni
+++ b/build/source_list.gni
@@ -398,7 +398,6 @@
     "src/quiche/spdy/core/spdy_protocol.h",
     "src/quiche/spdy/core/spdy_simple_arena.h",
     "src/quiche/spdy/core/zero_copy_output_buffer.h",
-    "src/quiche/web_transport/mock_web_transport.h",
     "src/quiche/web_transport/web_transport.h",
 ]
 quiche_core_srcs = [
@@ -852,6 +851,7 @@
     "src/quiche/quic/test_tools/web_transport_test_tools.h",
     "src/quiche/spdy/test_tools/mock_spdy_framer_visitor.h",
     "src/quiche/spdy/test_tools/spdy_test_utils.h",
+    "src/quiche/web_transport/test_tools/mock_web_transport.h",
 ]
 quiche_test_support_srcs = [
     "src/quiche/common/platform/api/quiche_test_loopback.cc",