I tried to run the currently unnecessary Mac 'darwin' check in 2.7 with
        if sys.platform == 'darwin':
            from lib-tk.test.runtktests import check_tk_availability
            # tkinter.test.suppport in 3.x
            try:
                check_tk_availability()
            except unittest.SkipTest:
                raise tk.TclError

This fails because 'lib-tk' is not a legal module name. There are workarounds in test/test_tk.py and test_ttk_guionly, but I decided not to bother with them now because I would rather work on moving the function to test.support (and perhaps the package could be given a legal name). If, before that is done, someone adds a Mac buildbot that will not run gui tests but requests that they be run anyway, the above could be added with the different changes needed for 2.7 and 3.x.