Issue 8273 is in some sense the opposite of this - it calls for blessing of everything in in test.support while this issue calls for wholesale "unblessing".  I don't think either approach is right.  While users should definitely never import test_* modules, I think test.regrtest and test.support are borderline enough that many users already started using it in their test suites.  (I am one of these users: on several projects, I actually renamed project-specific test directories to something else in order to be able to import stuff from test.)

I think the warning should be less than absolute.  Certainly users should not use things in test in their application code.  However, for use in test suites, it should be more of "use it at your own risk".