Add comments to test_updater.py and fix bug in signercli.py

Add comments to make it clearer how the self.Repository.refresh() call in test_5_all_targets() successfully returns the metadata for all the roles.  _mock_download_url_to_tempfileobj() is set up to allow mock downloads of the "all_role_paths" files in order.Relative filepaths were incorrectly calculated when
directories were specified as target paths in the --makedelegation command-line option for signercli.py.