Test different code paths
Different code paths are used in Pulp for the cases when artifacts are already present on the filesystem during sync and when they are not downloaded yet.
It probably makes sense to test 2 subsequent syncs of the same repo with immediate policy (doesn't matter which plugin) to be sure that second one is fine.
1. no content from repo A is downloaded
2. sync/publish repo A with download policy immediate
3. sync/publish repo A (or any other repo which has at least one artifact from repo A) again with download policy immediate
No failure in 2 shows that sync went fine when content was not present on the disk and in the DB.
No failure in 3 shows that sync went fine when content was already present on the disk and in the DB.