I have not really used unittest, so I only know to blindly copy what has been done. Hence I need help to do better.

Do you actually get gui tests for test_tk? When I run test/test_tk from Idle editor, and hence as __main__, I only get non-gui tests in spite of enable_gui being set True. Or maybe I just did not see them. (This is actually an improvement over the normal failure of test_tk on Windows. See #10652)

Is tkinter/test/runtktests properly called a test runner?
test/test_tk uses it to gather the tests to run:
support.run_unittest(
    *runtktests.get_tests(text=False, packages=['test_tkinter']))
It is used within the ttk tests also. As I suggested above, I do not really know if we really need the equivalent.

I strongly prefer idlelib/test since it will make developing **much** easier for me on Windows. Also, it would be part of the optional install of idlelib, as tkinter/test is for tkinter.

Adding test/test_idle will not be too much use until issue #10652 is resolved so it would actually run with -m test.