private <T> List<T> iteratorToList(final Iterator<T> iterator) {
        return StreamSupport.stream(
                        Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false)
                .collect(Collectors.toList());
    }
This test is testing something internal to [[KStreamKStreamJoin]], so we had to setup low-level api manually.
streams/src/test/java/org/apache/kafka/streams/kstream/internals/KStreamKStreamJoinTest.java