GeezerVison

This sets up for having a test_io._support which contains the I/O mocks for sharing between multiple io module test files.

The usual pattern is test.test_support.{topic}_helper, isn't it? Can you elaborate on the proposed _support module?

I was following what I saw in a couple instances (ex. test_string: https://github.com/python/cpython/blob/main/Lib/test/test_string/_support.py, test_zoneinfo: https://github.com/python/cpython/blob/main/Lib/test/test_zoneinfo/_support.py). There's also a couple of support.py (ex. https://github.com/python/cpython/blob/main/Lib/test/test_tkinter/support.py). No idea what is most common or being worked towards as layout.

The IO Mocks are only useful for testing the two io module implementations (_io C and _pyio Python) and have specific to io naming patterns that are currently required (ex. MockRawIO -> CMockRawIO, PyMockRawIO + load_tests injection https://github.com/python/cpython/blob/main/Lib/test/test_io.py#L5040-L5077). Things in test.support feel like they're useful for testing multiple Python modules which I don't think these mocks are.

Another approach here would be to make a mock_io.py which mirrors https://github.com/python/cpython/blob/main/Lib/test/mock_socket.py, which would leave test_io.py in the current location (and not need a __main__.py + __init__.py).

If only useful within test_io then a test_io._mock_io or similar seems appropriate, thanks for the explanation.

Could this get a skip news tag? (Test-only change)