I take the blame on documenting the handful of things in test.support; I thought it was a good idea to expose the handful of things that had an actual design to them. =)
But yes, we should probably shift to test.support to being private and simply exposing the stuff that we want to support to another module, although this stuff is not unittest-specific; its just testing utility functions. testlib or testutils would be a better place.
And we should make sure we are *very* comfortable supporting whatever we move over and that it has a good API as test.support has been a dumping ground for so long that most of that stuff should not get exposed (and honestly should get ripped out or cleaned up; TESTFN is just bad for instance).