What Xcode shows you:
What the actual underlying error, which is actually useful:
To see this error in detail, you have to jump through some hoops and print it in the debugger, when the Xcode test suite should just show you the more descriptive error in the first place:
I assume this is as simple as making the `.failureReason` more descriptive. I couldn't find where `DecodingError` was defined in `apple/swift-corelibs-foundation` or I would have checked.