Backport test_capi enhancements from Python 3.12 to Python 3.11

[vstinner]

I propose to copy Modules/_testcapi/ and Lib/test/test_capi/ from Python 3.12 to Python 3.11 to increase the test coverage in Python 3.11 and allow backporting test changes, since test changes should be backported to stable versions: Python 3.11 and 3.12.

[vstinner]

Example of test which would be nice to backport to Python 3.11: #110547 (comment)

[serhiy-storchaka]

$ git log --oneline 3.12 -- Modules/_testcapi/ Lib/test/test_capi/ | wc -l
108

[vstinner]

While it would be nice, I failed to find time to do that. So let's leave the code as it is. I just close the issue.