With @morty we decided that what this test is really checking that we don't get a duplicate iterator (now tested efficiently in `test_corebag_iterator_nonduplicate`). It was dominating the test runtime and it seemed like a round-about way of expressing a simple concept (the intent wasn't clear) so it is disabled by prefixing it with `_` pending removal (in case someone wants to shout - @scraperdragon ?)