Add tests for manifest lists and fix some issues encountered while testing

endpoints/v2/manifest.py

@@ -278,6 +278,7 @@ def _write_manifest(namespace_name, repo_name, tag_name, manifest_impl):
   if repository_ref is None:
     raise NameUnknown()
 
+  # Create the manifest(s) and retarget the tag to point to it.
   manifest, tag = registry_model.create_manifest_and_retarget_tag(repository_ref, manifest_impl,
                                                                   tag_name, storage)
   if manifest is None: