fix headers attribute of cxx_test in OSS

We forgot to make header paths work for cxx_test headers.

Test Plan: buck2 build //runtime/kernel/test:operator_registry_test

ghstack-source-id: cd88183
ghstack-comment-id: 2680189455
Pull Request resolved: pytorch/executorch#8662

Author: swolchok

shim_et/xplat/executorch/build/runtime_wrapper.bzl

@@ -268,6 +268,7 @@ def _cxx_test(*args, **kwargs):
     kwargs["deps"].append("//executorch/test/utils:utils")
 
     _patch_kwargs_cxx(kwargs)
+    env.patch_headers(kwargs)
     _patch_build_mode_flags(kwargs)
     _patch_test_compiler_flags(kwargs)