Any form of test you are just checking the way it is rigged and...

Any form of test you are just checking the way it is rigged and the way it reacts in various wind, wave and water conditions . Towing round things isn't the easiest but at this scale nothing too hard.

If it towed like a dog, wanted to tilt or veer etc they may consider ballasting it with water or changing the tow points adding a drogue rather than a trailing tug or any of many other combinations. I would assume they would do a controlled ballasting test in harbour and a emergency one so you know you could sink and recover it if required and all valves are operational along with all watertight areas are sealed under real conditions. They may be comfortable without doing these test live and wet and have just done them dry. Previous stability tests may have had to be redone if they had done modifications or the prior tests didn't match the computer models. .With any tow it all looks good until something goes wrong and experience of operators is paramount importance to recover the situation by making the right decisions for safety of crew,vessel and goods.