Non-serializable behavior in TestConcurrentIncrements

https://circleci.com/gh/cockroachdb/cockroach/9401

```
I1117 00:12:48.817836 865 storage/engine/rocksdb.go:132  closing in-memory rocksdb instance
    driver_test.go:483: unexpected max: 1
    driver_test.go:486: unexpected sum: 2
```

That is, both records are ending up with the value 1, instead of one being 1 and the other being 2.
