I closed #20567 after finishing what I intended to do 3 years ago. I decided make this a separate issue for trying Serhiy's suggestion of updata_idletasks. Pursuing 'no default root' is #24137.
3 other files also gave the warning, once, when run alone. 2 of the 4 'warned with the test suite in 3.5 and 3 of 4 in 3.6. Adding root.update or root.update_idletasks to all 4 fixed all warnings I know of for both versions. It appears to be a coincidence that the warnings consistently appeared where they did. I also added a note about this fix to README.txt.
There is no known problem in 2.7. I fixed 3.5 also because the warnings sometimes appear in buildbot reports, and they don't need the noise.