A bunch of tests were failing on Xcode 15 due to [a change in
Swift](https://github.com/apple/swift/issues/66466), because we relying
on being able to verify that the encoded `Data` saved to caches is
consistent.
Starting on Swift 5.9 `JSONEncoder` has optimizations that mean we can't
rely on it.
To fix this, this PR does 2 things:
- Changed `JSONEncoder.prettyPrinted` to have `.sortedKeys`
- Changed test to verify the decoded response `struct`, not the raw
encoded `Data`
Note that I didn't make `JSONEncoder.default` use `.sortedKeys` because
of the potential performance impact. `Encodable.prettyPrintedData` is
used in tests like `ReceiptFetcherTests` (which this PR fixes) and for
debug purposes, but `Encodable.encodedJSON` is used everywhere.
Facundo Menzella