Add const to resolve clang-tidy error.

```
-- The CXX compiler identification is Clang 15.0.7

/usr/bin/cmake -E __run_co_compile --tidy="/usr/bin/clang-tidy;--use-color;--warnings-as-errors=*;--extra-arg-before=--driver-mode=g++" --source=/__w/pybind11/pybind11/tests/test_class_sh_inheritance.cpp -- /usr/bin/c++ -DPYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD_IF_AVAILABLE -DPYBIND11_TEST_EIGEN -Dpybind11_tests_EXPORTS -I/__w/pybind11/pybind11/include -isystem /usr/include/python3.9 -isystem /__w/pybind11/pybind11/build/_deps/eigen-src -Os -DNDEBUG -fPIC -fvisibility=hidden -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -flto=thin -std=c++17 -o CMakeFiles/pybind11_tests.dir/test_class_sh_inheritance.cpp.o -c /__w/pybind11/pybind11/tests/test_class_sh_inheritance.cpp
/__w/pybind11/pybind11/tests/pure_cpp/smart_holder_poc_test.cpp:264:30: error: pointer parameter 'raw_ptr' can be pointer to const [readability-non-const-parameter,-warnings-as-errors]
        new int(19), [](int *raw_ptr) { delete raw_ptr; });
                             ^
                        const
```

tests/pure_cpp/smart_holder_poc_test.cpp

@@ -260,8 +260,8 @@ TEST_CASE("from_unique_ptr_with_deleter+as_lvalue_ref", "[S]") {
 }
 
 TEST_CASE("from_unique_ptr_with_std_function_deleter+as_lvalue_ref", "[S]") {
-    std::unique_ptr<int, std::function<void(int *)>> orig_owner(
-        new int(19), [](int *raw_ptr) { delete raw_ptr; });
+    std::unique_ptr<int, std::function<void(const int *)>> orig_owner(
+        new int(19), [](const int *raw_ptr) { delete raw_ptr; });
     auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
     REQUIRE(orig_owner.get() == nullptr);
     REQUIRE(hld.as_lvalue_ref<int>() == 19);