[3.14] gh-135906: Test more internal headers in test_cext/test_cppext (#144758)

* gh-141563: Enable test_cppext internal C API tests on macOS (#144711)

Build the C API in C++11 mode on macOS.

(cherry picked from commit c6e418d1744aed95a6f25d22565204649dde29c7)

* gh-135906: Test more internal headers in test_cext/test_cppext (#144751)

(cherry picked from commit b488f338cf058f46cbf0255023ca1c1669b0eb44)