ctypes.Union & ctypes.Structure: common features should be tested on both types

[encukou]

Currently, Lib/test/test_ctypes/test_structure.py is very long, but Lib/test/test_ctypes/test_union.py only has recent additions.
A lot of the Structure tests apply to Union as well, and should be run on both. My plan is:

• Move tests of common features to Lib/test/test_ctypes/test_structunion.py, and use the “common base class + two subclasses” pattern used for tests of similar classes
• Keep the Structure-specific tests in Lib/test/test_ctypes/test_structure.py

Linked PRs

• gh-124570: ctypes: Run some Structure tests on Union as well #124976

[rruuaanng]

It seems like the workload is quite heavy. I don't think it's necessary just to organize an almost stable and unchanging module.

[encukou]

But ctypes isn't stable and unchanging. In 3.14 the struct/union packing algorithm is replaced and moved to Python code. That on its own deserves better Union tests.