In particular, check buggy behavior around embedded quoted JSON
and Postgres versions, compare
https://www.postgresql.org/message-id/D6921B37-BD8E-4664-8D5F-DB3525765DCD%40vllmrt.net
This adds version-bounded tests for the handling of quoted JSON,
and a pending test that documents the assumption that Postgres >=10
intends to parse quoted JSON as a string (similar to how jsonb works
now).