Unstable unit test sstable::tests::test_randomized_key

```
running 1 test
test sstable::tests::test_randomized_key ... FAILED

failures:

---- sstable::tests::test_randomized_key stdout ----
thread 'sstable::tests::test_randomized_key' panicked at 'called `Option::unwrap()` on a `None` value', src/db/iterator.rs:387:43
```