I think this can be simplified and easier to read as something like:

    def test_combinations_with_replacement(self):
        def work(it):
            while True:
                try:
                    next(it)
                except StopIteration:
                    break

        threading_helper.run_concurrently(work, nthreads=6, args=[combinations_with_replacement(tuple(range(2)), 2)])

• I think it's generally not worth the CI time to run multiple iterations in the hopes of making bugs more likely. TSan is pretty good at catching these things even if it doesn't crash. If I remove the critical sections, most runs report a data race with TSan.
• All or nearly all the tests have the same def work(). We can extract that too
• I don't think we need the @threading_helper.reap_threads. threading_helper.run_concurrently handles that.

The diff is now larger, but the code is indeed much simpler. I kept the @threading_helper.reap_threads for now, claude things we still need it in case of exceptions (I will double check later).

For all the tests I reduced the number of iterations. We could remove the iterations altogether, but I kept it to the test the concurrent exhaustion of the iterator (which is different from the concurrent iteration part). If needed I can remove the multiple iterations altogether.