> You left this issue number off your tkinter test updates, such as
> f6f098bdb843.

This is minor change. Actually, it should be done yet in issue22236, I had 
just missed it, because these warnings was produced only in 2.7.

> By using .update_idletasks instead of .update, are you
> assuming that their are no user events, or that they should be ignormed?

There are not only "user" events. When you create or configure some widgets, 
some actions is not executed immediately (in particular changing a theme), but 
they are deferred to the next call of update() or update_idletasks(). When you 
call update_idletasks(), these harmless events are quickly handled without 
errors. This is only known me way to clear events queue.

> I was wondering whether it would be Ok, if not a good idea, to run tests,
> and maybe Idle itself, with NoDefaultRoot.

I support it. There are some places in IDLE or tests which should be fixed, 
they are used default master.

> I cannot use exactly the same design as AbstractTkTest since
> I sometimes do other things in setUp/tearDownClass, but I could define
> create_test_root() and delete_test_root() functions in a support file.

See Lib/tkinter/test/test_tkinter/test_images.py. Image tests do  other things 
in setUpClass/tearDownClass, but calls parent's method too.