tests: consistent_snapshot tests in test_updater_ng.py are broken

`test_refresh_on_consistent_targets()` does not actually try to refresh any metadata apart from root and timestamp (and it couldn't because the files don't exist in the repository). The test only passes because client already has all the metadata in the metadata cache.

I'm not sure it makes sense to fix this before #1444 but I'm filing so we don't make this mistake again.