Summary:
The patch adds checks to the
`{NonBatchedOps,BatchedOps,CfConsistency}StressTest::TestPrefixScan` methods
to make sure the wide columns exposed by the iterators are as expected (based on
the value base encoded into the iterator value). It also makes some code hygiene
improvements in these methods.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10786

Test Plan:
Ran some simple blackbox tests in the various modes (non-batched, batched,
CF consistency).

Reviewed By: riversand963

Differential Revision: D40163623

Pulled By: riversand963

fbshipit-source-id: 72f4c3b51063e48c15f974c4ec64d751d3ed0a83