catalog_control_test.cpp and kv_drop_pending_ident_reaper_test.cpp both create mock storage engines. It would be useful to move and merge those implementations into a mock storage engine file, so that other tests can utilize the same implementation rather than continuing to write mocks per file.

We should also explore why mocks were created instead of using the devnull storage engine. From a cursory examination, it seems arguable that the dev null engine is helpful for perf. A mock that does special actions, like one of the mocks does, would not be good for perf testing. Perhaps update the devnull engine to specify it is only used for perf reasons now? If that is true.