Summary:
The behaviour of WBWI has changed when calling newIteratorWithBase when overwrite is set to true or false. This PR simply adds tests to assert the new correct behaviour.

Closes https://github.com/facebook/rocksdb/issues/7370
Closes https://github.com/facebook/rocksdb/pull/8134

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9107

Reviewed By: pdillinger

Differential Revision: D32099475

Pulled By: mrambacher

fbshipit-source-id: 245f483f73db866cc8a51219a2bff2e09e59faa0