[issue25591] refactor imaplib tests

[R. David Murray]

R. David Murray added the comment:

Revised patch LGTM.  Once 3.6 is out the door I'll commit it, though you may need to remind me.

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stage: patch review -> commit review
versions: +Python 3.7 -Python 3.6

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<http://bugs.python.org/issue25591>
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