pythongh-138013: Move I/O tests to test_io (python#138365)

Centralize `io` tests into the `test_io` module so they are easier to
find and work on. This will make it easier to split `test_general` which
takes 30+ seconds in a debug build on my machine.

This renames `test_bufio` to be `test_bufferedio` so that it matches
the implementation file name (`bufferedio.c`).

Validation performed:
Tests are run in parallel after change:

```bash
./python.exe -m test test_io  -uall,largefile,extralargefile -M12G -j8
```

Docstring reformat in `test_io/__init__.py` looks reasonable:

```python
>>> import test.test_io
>>> help(test.test_io)
```

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: cmaloney

Lib/test/libregrtest/findtests.py

@@ -24,6 +24,7 @@
     "test_future_stmt",
     "test_gdb",
     "test_inspect",
+    "test_io",
     "test_pydoc",
     "test_multiprocessing_fork",
     "test_multiprocessing_forkserver",

Lib/test/test_io/__init__.py

@@ -1,3 +1,26 @@
+"""Tests for the io module and its implementations (_io and _pyio)
+
+Tests are split across multiple files to increase
+parallelism and focus on specific implementation pieces.
+
+* test_io
+  * test_bufferedio - tests file buffering
+  * test_memoryio - tests BytesIO and StringIO
+  * test_fileio - tests FileIO
+  * test_file - tests the file interface
+  * test_general - tests everything else in the io module
+  * test_univnewlines - tests universal newline support
+  * test_largefile - tests operations on a file greater than 2**32 bytes
+    (only enabled with -ulargefile)
+* test_free_threading/test_io - tests thread safety of io objects
+
+.. attention::
+   When writing tests for io, it's important to test both the C and Python
+   implementations. This is usually done by writing a base test that refers to
+   the type it is testing as an attribute. Then it provides custom subclasses to
+   test both implementations. This directory contains lots of examples.
+"""
+
 import os
 from test.support import load_package_tests
 

Lib/test/test_bufio.py Lib/test/test_io/test_bufferedio.pyLib/test/test_bufio.py renamed to Lib/test/test_io/test_bufferedio.py

@@ -1,24 +1,7 @@
-"""Unit tests for the io module."""
-
-# Tests of io are scattered over the test suite:
-# * test_bufio - tests file buffering
-# * test_memoryio - tests BytesIO and StringIO
-# * test_fileio - tests FileIO
-# * test_file - tests the file interface
-# * test_io.test_general - tests everything else in the io module
-# * test_univnewlines - tests universal newline support
-# * test_largefile - tests operations on a file greater than 2**32 bytes
-#     (only enabled with -ulargefile)
-# * test_free_threading/test_io - tests thread safety of io objects
-
-################################################################################
-# ATTENTION TEST WRITERS!!!
-################################################################################
-# When writing tests for io, it's important to test both the C and Python
-# implementations. This is usually done by writing a base test that refers to
-# the type it is testing as an attribute. Then it provides custom subclasses to
-# test both implementations. This file has lots of examples.
-################################################################################
+"""General tests for the io module.
+
+New tests should go in more specific modules; see test_io/__init__.py
+"""
 
 import abc
 import array