gh-141563: Fix test_cppext on macOS (#144685)

Don't test internal header files including mimalloc on macOS since
mimalloc emits compiler warnings:

  In file included from extension.cpp:21:
  In file included from Include/internal/pycore_backoff.h:15:
  In file included from Include/internal/pycore_interp_structs.h:15:
  In file included from Include/internal/pycore_tstate.h:14:
  In file included from Include/internal/pycore_mimalloc.h:43:
  Include/internal/mimalloc/mimalloc.h:464:85: error: defaulted
  function definitions are a C++11 extension
  [-Werror,-Wc++11-extensions]
    mi_stl_allocator() mi_attr_noexcept = default;
                                                                                      ^
  Include/internal/mimalloc/mimalloc.h:465:85: error: defaulted
  function definitions are a C++11 extension
  [-Werror,-Wc++11-extensions]
    mi_stl_allocator(const mi_stl_allocator&) mi_attr_noexcept = default;

Log also CXX and CXXFLAGS env vars in test_cppext. Log also CPPFLAGS
in test_cext.