Add backward-compatibility tests for json serialization

[Strilanc]

Currently we do some round trip testing, but there's nothing that checks that we aren't changing the json serialization from version to version in a way that breaks the ability to parse previously serialized things.

There are two main criteria I have in mind:

1. It should somehow be clear to developers that it is not okay to change the test data json, since that would defeat the purpose of a compatibility test.

2. It should re-use the mechanism in json_test for ensuring we are testing every exposed class.