[3.11] gh-108303: Add Lib/test/test_cppext/ sub-directory (#108325) (#108336)

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