I just noticed another similar test dependency, this time between test_idle and test_tix.  If run with -j set, the problem is not seen.  This is still present in 3.6.0a4:

$ python3.6 -m test -uall -w test_idle test_tix
Run tests sequentially
0:00:00 [1/2] test_idle
can't invoke "event" command:  application has been destroyed
    while executing
"event generate $w <<ThemeChanged>>"
    (procedure "ttk::ThemeChanged" line 6)
    invoked from within
"ttk::ThemeChanged"
0:00:02 [2/2] test_tix
test test_tix crashed -- Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/test/libregrtest/runtest.py", line 167, in runtest_inner
    the_module = importlib.import_module(abstest)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 996, in _gcd_import
  File "<frozen importlib._bootstrap>", line 979, in _find_and_load
  File "<frozen importlib._bootstrap>", line 968, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 667, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/test/test_tix.py", line 11, in <module>
    from tkinter import tix, TclError
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/tkinter/tix.py", line 30, in <module>
    from tkinter import _cnfmerge, _default_root
ImportError: cannot import name '_default_root'

test_tix failed
1 test OK.
1 test failed:
    test_tix
Re-running failed tests in verbose mode
Re-running test 'test_tix' in verbose mode
test test_tix crashed -- Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/test/libregrtest/runtest.py", line 167, in runtest_inner
    the_module = importlib.import_module(abstest)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 996, in _gcd_import
  File "<frozen importlib._bootstrap>", line 979, in _find_and_load
  File "<frozen importlib._bootstrap>", line 968, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 667, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/test/test_tix.py", line 11, in <module>
    from tkinter import tix, TclError
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/tkinter/tix.py", line 30, in <module>
    from tkinter import _cnfmerge, _default_root
ImportError: cannot import name '_default_root'

1 test failed again:
    test_tix
Total duration: 0:00:03