[issue18273] Simplify calling and discovery of json test package

[Ezio Melotti]

Ezio Melotti added the comment:

I like the patch.
Can you make "./python Lib/test/test_json/" work too?  Currently it doesn't seem to work (it works for e.g. "./python Lib/test/test_email/").

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stage:  -> patch review

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18273>
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