[issue35124] Add style guide for unit tests

[STINNER Victor]

STINNER Victor <vstinner at redhat.com> added the comment:

Context: https://github.com/python/cpython/pull/8548#pullrequestreview-166712031

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nosy: +vstinner

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35124>
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