gh-138013: Move test.test_io to be a module

This sets up for having a test_io._support which contains the I/O mocks
for sharing between multiple `io` module test files. Also starts a path
for all the `io` module tests to live in a single module for simpler
running rather than requiring knowledge of the list of `io` tests which
lives in `test_io`.

Keeps `python -m test test_io -uall -M8g` and `python -m test.test_io` running the
same set of tests as before.

Author: cmaloney

Lib/test/test_io/__init__.py

@@ -0,0 +1,5 @@
+import os
+from test.support import load_package_tests
+
+def load_tests(*args):
+    return load_package_tests(os.path.dirname(__file__), *args)

Lib/test/test_io/__main__.py

@@ -0,0 +1,4 @@
+from . import load_tests
+import unittest
+
+unittest.main()

Lib/test/test_io.py renamed to Lib/test/test_io/test_general.py

@@ -5,7 +5,7 @@
 # * test_memoryio - tests BytesIO and StringIO
 # * test_fileio - tests FileIO
 # * test_file - tests the file interface
-# * test_io - tests everything else in the io module
+# * 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)

Makefile.pre.in

@@ -2669,6 +2669,7 @@ TESTSUBDIRS=	idlelib/idle_test \
 		test/test_importlib/source \
 		test/test_inspect \
 		test/test_interpreters \
+		test/test_io \
 		test/test_json \
 		test/test_module \
 		test/test_multiprocessing_fork \