The external dependency upon vstorage data format is not captured in unit tests

[gibson042]

@agoric/casting interacts directly with the vstorage KVStore using the /store/{storeName}/key cosmos-sdk escape hatch, making assumptions about the structure of keys and the structure of values. This creates an inherent constraint on our ability to change such details in vstorage, and that constraint should be captured in unit tests referencing the casting code (similar to what we did in #8538). Likewise, the casting code should reference documentation of the format rather than exercising in opaque deconstruction.