Fix mod_per_interpreter_gil* output directory on Windows/MSVC

On Windows with MSVC (multi-configuration generators), CMake uses
config-specific properties like LIBRARY_OUTPUT_DIRECTORY_DEBUG when
set, otherwise falls back to LIBRARY_OUTPUT_DIRECTORY/<Config>/.

The main test modules (pybind11_tests, etc.) correctly set both
LIBRARY_OUTPUT_DIRECTORY and the config-specific variants (lines
517-528), so they output directly to tests/.

However, the mod_per_interpreter_gil* modules only copied the base
LIBRARY_OUTPUT_DIRECTORY property, causing them to be placed in
tests/Debug/ instead of tests/.

This mismatch caused test_import_in_subinterpreter_concurrently and
related tests to fail with ModuleNotFoundError on Windows Python 3.14,
because the test code sets sys.path based on pybind11_tests.__file__
(which is in tests/) but tries to import mod_per_interpreter_gil_with_singleton
(which ended up in tests/Debug/).

This bug was previously masked by @pytest.mark.xfail decorators on
these tests. Now that the underlying "Duplicate C++ type registration"
issue is fixed and the xfails are removed, this path issue surfaced.

The fix mirrors the same pattern used for main test targets: also set
LIBRARY_OUTPUT_DIRECTORY_<CONFIG> for each configuration type.

Author: rwgk

tests/CMakeLists.txt

@@ -592,6 +592,14 @@ if(NOT PYBIND11_CUDA_TESTS)
   foreach(mod IN LISTS PYBIND11_MULTIPLE_INTERPRETERS_TEST_MODULES)
     set_target_properties("${mod}" PROPERTIES LIBRARY_OUTPUT_DIRECTORY
                                               "${pybind11_tests_output_directory}")
+    # Also set config-specific output directories for multi-configuration generators (MSVC)
+    if(DEFINED CMAKE_CONFIGURATION_TYPES)
+      foreach(config ${CMAKE_CONFIGURATION_TYPES})
+        string(TOUPPER ${config} config)
+        set_target_properties("${mod}" PROPERTIES LIBRARY_OUTPUT_DIRECTORY_${config}
+                                                  "${pybind11_tests_output_directory}")
+      endforeach()
+    endif()
   endforeach()
 
   if(PYBIND11_TEST_SMART_HOLDER)