[tests] Backport test_cext and test_cppext (C API tests) from main to the 3.12 branch

[vstinner]

The C API is not well tested in the 3.12 branch. I propose to backport test_cext and test_cppext from the main branch to the 3.12 branch.

Example of recent C++ regression: #127902

Linked PRs

• [3.12] gh-127906: Declare timeval struct in pytime.h on Windows #127908
• [3.12] gh-127906: Backport test_cext from the main branch #127912
• [3.13] gh-127906: Backport test_cppext changes from main #127914
• [3.12] gh-127906: Backport test_cppext from the main branch #127915
• gh-127906: Test the limited C API in test_cppext #127916
• [3.13] gh-127906: Test the limited C API in test_cppext (GH-127916) #127919
• [3.12] gh-127906: Test the limited C API in test_cppext (GH-127916) #127920
• [3.13] gh-127906: Backport test_cext changes from the main branch #127923
• [3.12] gh-127906: Skip limited C API test_cext tests if Py_TRACE_REFS #127993
• [3.13] gh-127906: Fix Py_BUILD_ASSERT_EXPR() on Windows #127994
• gh-127906: Add missing sys import to test_cppext #132902
• [3.13] gh-127906: Add missing sys import to test_cppext (GH-132902) #132904
• [3.12] gh-127906: Add missing sys import to test_cppext (GH-132902) #132905

[vstinner]

On the 3.12 branch, test_cext currently fails with these warnings:

  C:\victor\python\3.12\include\cpython/unicodeobject.h(205): error C2220: the following warning is treated as an error
  C:\victor\python\3.12\include\cpython/unicodeobject.h(205): warning C4100: '_unused_op': unreferenced formal parameter
  C:\victor\python\3.12\include\cpython/unicodeobject.h(395): warning C4100: '_unused_op': unreferenced formal parameter
  C:\victor\python\3.12\include\cpython/pytime.h(192): warning C4115: 'timeval': named type definition in parentheses
  extension.c(22): warning C4100: '_unused_module': unreferenced formal parameter
  extension.c(44): warning C4100: '_unused_module': unreferenced formal parameter

I backported MSVC support in Py_UNUSED() to 3.12 to fix unicodeobject.h: PR gh-127907.

[vstinner]

C:\victor\python\3.12\include\cpython/pytime.h(192): warning C4115: 'timeval': named type definition in parentheses

I wrote #127908 to fix this pytime.h warning.