fix(tests): Install multiple-interpreter test modules into wheel

The `mod_per_interpreter_gil`, `mod_shared_interpreter_gil`, and
`mod_per_interpreter_gil_with_singleton` modules were being built
but not installed into the wheel when using scikit-build-core
(SKBUILD=true). This caused iOS (and potentially Android) CIBW
tests to fail with ModuleNotFoundError.

Root cause analysis:
- The main test targets have install() commands (line 531)
- The PYBIND11_MULTIPLE_INTERPRETERS_TEST_MODULES were missing
  equivalent install() commands
- For regular CMake builds, this wasn't a problem because
  LIBRARY_OUTPUT_DIRECTORY places the modules next to pybind11_tests
- For wheel builds, only targets with explicit install() commands
  are included in the wheel

This issue was latent until commit fee2527d changed the test imports
from `pytest.importorskip()` (graceful skip) to direct `import`
statements (hard failure), which exposed the missing modules.

Failing tests:
- test_multiple_interpreters.py::test_independent_subinterpreters
- test_multiple_interpreters.py::test_dependent_subinterpreters

Error: ModuleNotFoundError: No module named 'mod_per_interpreter_gil'

tests/CMakeLists.txt

@@ -602,6 +602,12 @@ if(NOT PYBIND11_CUDA_TESTS)
     endif()
   endforeach()
 
+  if(SKBUILD)
+    foreach(mod IN LISTS PYBIND11_MULTIPLE_INTERPRETERS_TEST_MODULES)
+      install(TARGETS "${mod}" LIBRARY DESTINATION .)
+    endforeach()
+  endif()
+
   if(PYBIND11_TEST_SMART_HOLDER)
     foreach(mod IN LISTS PYBIND11_MULTIPLE_INTERPRETERS_TEST_MODULES)
       target_compile_definitions(