Problem/Motivation
While writing tests for #2875867: Add per-bundle unpublished content permissions I noticed that the EntityTestWithBundle entity isn't revisionable even though it extends ContentEntityBase, which implements RevisionableInterface.
I'm not sure if it's intentional that this entity type doesn't declare a revisionable entity key or not, but if it's just an oversight, then it probably should be revisionable.
Proposed resolution
Declare the "revision" entity key.