(The fix was incomplete because 'delete' in the except clause should be 'remove'.) I finally realized that I could test the except clause by raising TclError explicitly just before it.

I reverted both changes because they do not seem to work, even with the additional fix (not committed). With or without patches,
python_d -m test -v test_idle  # runs 30 tests, as expected
python_d -m test -v -ugui test_idle  # run 40 tests, as expected
After raising TclError, the latter still runs 40 tests, which I did not expect.

It seems that 'gui' is not being removed from use_resources. Since debug print()s in test_idle do not show up on the console, I cannot easily see what is going on.