restore: flush storage at every loop
In the production-like setting, we've deployed swh-alter with a buffered storage for the restore process. If the buffer storage is not flushed (at least on exit), it just loses the added objects.
To honor the swh-alter chunking mechanism, we call storage.flush() after every call to an add function. We make sure to run the recovery_bundle tests with a buffered storage to trigger the same code paths.