I have a behavior that registers the asset, actually I think I found the problem.
Since the test stubs the AssetManager class it never publishes the files. This in turn never sets baseUrl in the asset. When I set the alias it is actually NULL.
Yii::setAlias('@tater', $tater_asset->baseUrl);
I can do a work around, but this is poor coding methodology.
Actually after reading some things about the latest versions of codeception it almost feels a little broken with yii2 at the moment. Some more documentation and fixes on how to do proper testing would be appreciative, they seem a bit light and un-intuitive at the moment.