test_setupapp calls setupApp with body 
    if isAquaTk():
        hideTkConsole(root)
        overrideRootMenu(root, flist)
        addOpenEventSupport(root, flist)
        fixb2context(root)
isAquaTk should be True exactly twice, and it failed both times.  The overall code is definitely not re-entrant  On Carbon, 'application' in inserted before 'file' and becomes menudef[0].  The puzzle to me is the failure on what should be the first call.

Until that is figured out, you could try decorating test_setupapp with
    @mock.patch(idlelib.macosx.setupApp)
to see what, if anything, fails next.

I have access to a small Macbook Air with 11" diag. screen -- probably too small for my eyes for routine use but I should be able to run test_idle (especially post push) and do occasional exploratory editing, as needed here.  I will try to learn enough about how to use it to follow the devguide instructions on setting up a repository.