Consolidate fake etcd KVs #4490

While doing the rebase I noticed that we currently have three identical implementations of a fake KV. These should be consolidated into a new package. I just didn't want to do it in the big rebase PR. Will pick this one up afterwards.

For future Simon

M       pkg/objectcount/scanner_test.go
M       pkg/reconciler/migration/logicalclustermigration/datacleanup_test.go
M       pkg/server/migrationdump/server_test.go