Terry, b10a312f6d00 appears to fix most of the issue, that is, if I insert a deliberate failure into another IDLE test and use -w without -j, the four repeatable errors I identified above no longer occur.  Thanks.  However, the fifth, timing-dependent error still occurs during the -w rerun perhaps ever 5th run or so.  It might be worth pursuing to help squash other intermittent results:

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ERROR: setUpClass (idlelib.idle_test.test_autocomplete.AutoCompleteTest)
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Traceback (most recent call last):
  File "/py/dev/3x/root/fwd_macports/Library/Frameworks/pytest_10.12.framework/Versions/3.6/lib/python3.6/idlelib/idle_test/test_autocomplete.py", line 34, in setUpClass
    macosx.setupApp(cls.root, None)
  File "/py/dev/3x/root/fwd_macports/Library/Frameworks/pytest_10.12.framework/Versions/3.6/lib/python3.6/idlelib/macosx.py", line 245, in setupApp
    overrideRootMenu(root, flist)
  File "/py/dev/3x/root/fwd_macports/Library/Frameworks/pytest_10.12.framework/Versions/3.6/lib/python3.6/idlelib/macosx.py", line 215, in overrideRootMenu
    del mainmenu.menudefs[-1][1][0]
IndexError: list assignment index out of range