[issue16898] Fix test discovery for test_bufio.py

[Zachary Ware]

New submission from Zachary Ware:

Version 2 replaces test_main() with unittest.main().

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Added file: http://bugs.python.org/file28655/test_bufio_fix.v2.diff

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16898>
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