tests: Move non-filesystem io tests to basics dir with io_ prefix.

tests/basics/io_buffered_writer.py

tests/basics/io_buffered_writer.py.exp

tests/basics/io_bytesio_cow.py

tests/basics/io_bytesio_ext.py

tests/basics/io_bytesio_ext2.py

tests/basics/io_bytesio_ext2.py.exp

tests/basics/io_iobase.py

tests/basics/io_stringio1.py

tests/basics/io_stringio_with.py

tests/basics/io_write_ext.py

tests/basics/io_write_ext.py.exp