Change assertList to fail faster

While testing a store I'm developing, I encountered the test suite apparently hanging during the list (and related) tests. Turned out this was because the store was returning too many results (because it was inserting new records instead of updating), and the `assertList` method would then get stuck in an infinite loop of calling `next()` to get more records because the actual length was not equal to the expected length. 

Eventually, the tests timeout, so a failure is a failure. But this probably a more correct test assertion, and it yields a more informative error. Plus it fails faster.