[issue35668] Improve test coverage for idlelib

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

I like having multiple commits with explanatory messages.  They get squashed when merging into master.  Unless I get distracted and forget, I will squash the messages.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35668>
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