Right you are, Chris. v4 has a comment added to regrtest.runtest_inner pointing back to this issue.
Also in v4, ReapedSuite has been moved to test.support. At least one other test module (test_pydoc) uses the same idiom as test_concurrent_futures, and so could use this suite for the same effect. Several others use at least one of reap_threads (or its component pieces) or reap_children in test_main, and I believe those could also use ReapedSuite for simplicity's sake. Used in its current form, it shouldn't cause any issues other than perhaps an extra couple of function calls.