Add default implementation of quiche_test.h

Note that since this uses quic_flags_list.h, it is currently not possible to build this inside google3.  I did run Copybara and test the result manually.

PiperOrigin-RevId: 449026859
diff --git a/build/source_list.bzl b/build/source_list.bzl
index 2a2bb10..855c33d 100644
--- a/build/source_list.bzl
+++ b/build/source_list.bzl
@@ -1399,10 +1399,12 @@
     "common/platform/default/quiche_platform_impl/quiche_file_utils_impl.cc",
 ]
 default_platform_impl_test_support_hdrs = [
+    "common/platform/default/quiche_platform_impl/quiche_test_impl.h",
     "common/platform/default/quiche_platform_impl/quiche_test_loopback_impl.h",
     "common/platform/default/quiche_platform_impl/quiche_thread_impl.h",
 ]
 default_platform_impl_test_support_srcs = [
+    "common/platform/default/quiche_platform_impl/quiche_test_impl.cc",
     "common/platform/default/quiche_platform_impl/quiche_test_loopback_impl.cc",
 ]
 load_balancer_hdrs = [