I think the issue of whether/how to test Python2 idle should be discussed on idle-dev.  One strategy is to make the tests backward compatible (so no mock).  Another possible strategy is to have an extra bit of test framework for IDLE in 2.7 that copies the tests and mock from another location in order to run tests against 2.7.  That is, the tests wouldn't be an official part of the 2.7 repro and would not be run by the buildbots.  The reason for doing that would be to allow the Python3 tests to be as rich as possible while still doing some testing on 2.7.  But that's just a thought experiment, as I said I think the full strategy should be hashed out on idle-dev (I'm not a member of that list :)