Description
Summary
We should tests the types of all formats, to ensure that the module system doesn't let more or less through than what the format generators expect.
Motivation
I stumbled upon an issue earlier with pkgs.formats.hocon, where it didn't allow me to set _includes as specified in the docs, because the nixos module wouldn't accept it's type. The _includes functionality is well tested through the format generator, but there is no test to ensure that the type agrees. And AFAIK, none of the format types have been tested in isolation (although many are being indirectly tested in nixosTests through modules that use the format type).
I think it would be a good addition to create such tests for all formats, maybe by leveraging lib.evalModules.
Maintainer pings: @h7x4 @Stunkymonkey