Summary:
Right now, db_stress doesn't cover SeekForPrev(). Add the coverage, which mirrors what we do for Seek().
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6022
Test Plan: Run "make crash_test". Do some manual source code hack to simular iterator wrong results and see it caught.
Differential Revision: D18442193
fbshipit-source-id: 879b79000d5e33c625c7e970636de191ccd7776c