gh-138013: Move I/O tests to test_io (#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>