[3.12] gh-108303: Add Lib/test/test_cppext/ sub-directory (GH-108325) (#108328)

gh-108303: Add Lib/test/test_cppext/ sub-directory (GH-108325)

* Move test_cppext to its own directory
* Rename setup_testcppext.py to setup.py
* Rename _testcppext.cpp to extension.cpp
* The source (extension.cpp) is now also copied by the test.
(cherry picked from commit 21dda09600848ac280481f7c64f8d9516dc69bb2)

Co-authored-by: Victor Stinner <vstinner@python.org>