$ ./python -m unittest -v test.test_{idle,tix}
. . .
test_tix (unittest.loader._FailedTest) ... ERROR

======================================================================
ERROR: test_tix (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: test_tix
Traceback (most recent call last):
  File "/media/disk/home/proj/python/cpython/Lib/unittest/loader.py", line 153, in loadTestsFromName
    module = __import__(module_name)
  File "/media/disk/home/proj/python/cpython/Lib/test/test_tix.py", line 11, in <module>
    from tkinter import tix, TclError
  File "/media/disk/home/proj/python/cpython/Lib/tkinter/tix.py", line 30, in <module>
    from tkinter import _cnfmerge, _default_root
ImportError: cannot import name '_default_root'

Without test_idle, test_tix is skipped for me:

$ ./python -m unittest -v test.test_tix
test_tix_available (test.test_tix.TestTix) ... skipped 'Tix not available'

Reverting to before revision 064b29dde096 (Issue 24137) also fixes the failure.