This is great. Good to see standardized testing improvements to the pg_regress add-on. YAML does better than JSON when it comes to readability. To an extent, TOML as well.
JSON is unambiguous and whitespace agnostic. There's only one way to do things. It's far more readable.
JSON as-is has fewer types and some things are better realized with the JSONNET way than stock.
Either way, JSON5 has been around for years and is a logical update because it brings parity with ECMAScript.
https://mit.cs.uchicago.edu/haohsiang/topics-auction/tree/02....
When I read "gbq enabled: yes" I don't worry about whether it's a string or a boolean when reading it, I worry about whether it's a string or a boolean when I want to change it and it gets parsed again.