Oh, no support for Python2?
I think, it is too old, but still needs bug-fix supports.
IDLE for Python2 is really buggy.
For example, unicode problems in my environment. http://bugs.python.org/issue17348
It might be GUI related problem.

By the way, your proposal seemed good to me.
My proposal is almost same.
Using unittest module, make directory such as idlelib/test and write unittest there.

But I want to include Python2 and GUI support.
(So, there are unittset.mock problem)

Of cource, it is very import to concentrate on one thing (i.e Python3).
I cannot deside whether to drop Python2 support now.

I considered PEP 434 premit me writing unittest for Python2.
http://www.python.org/dev/peps/pep-0434/

How do you think?