writtenBytesComeBackExactly

The write/read round trip, plus the fact that a StoreKey addresses by its name and not by object identity.

The second assertion looks like a formality and is not: StoreKey is a value class, so on some targets it is erased to a bare String and on others it is boxed — a backend keying a map on the wrapper rather than on StoreKey.name can work on one platform and fail on another, which is precisely the kind of difference four independently-written backends produce.