[issue3643] Add more checks to testcapi

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

@pitrou: This issue is not critical. It's not a bug, it's an 
enhancement since _testcapi :-)

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue3643>
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