Some objects (directories, origins, releases, revisions) will not be added into the storage
if they are provided as generator parameters to the `Storage::*_add` methods instead of lists.
This is why the `swh-vault` tests are currently failing as since the removal of the retry
mechanism in `swh-loader-core`, generators are provided as parameters to the `Storage::*_add`
methods while previously lists of objects were provided.
As a simple fix, ensure to transform generators into lists before processing the objects.