[issue19590] Use specific asserts in test_email

[R. David Murray]

R. David Murray added the comment:

Additional changes look good to me.

Unless (pun intended) Barry objects, I think I'll come down in favor of backporting all of these changes.  The tipping point is that I've always found myself experiencing cognitive dissonance reading the 'unless' calls, since they are the "opposite sense" of all of the other 'assert' style calls.  Having all these tests be the same in all active branches will also help reduce cognitive dissonance while debugging failures :)

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19590>
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