Centralize I/O tests #161458

Feature or enhancement

Currently I/O tests largely live in test_io but free-threaded tests and some interrupt tests live other places. Move them all to live in test_io.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-161458: Move test_winconsoleio and test_file_eintr into test_io #161460
• [3.15] gh-161458: Move test_winconsoleio and test_file_eintr into test_io (GH-161460) #161523