If things are fixed/added/improved on 3.x, there should be tests for them, and if they are backported on 2.7, tests should be backported as well.
If mock makes testing easier, I think it would be acceptable to use it and then have IDLE devs install a 2.7 mock and use it to run all the tests.  The tests on 2.7 could use skip decorators to be skipped if mock is missing, without having to keep these tests separate from the rest.  If necessary a script to install mock on 2.7 could be provided, and possibly used by the buildbots too.

This means that the initial framework should be backported, otherwise it won't be possible to backport any of tests that are going to use it.
FWIW the attached patch should use unittest.main() instead of test_main+run_unittest.