test: add lint rule to enforce trailing commas

Only activated on some subfolders to minimize the diff, ideally this rule would be applied gradually to the entire codebase in follow-up commits.

There have been previous attempts at imposing trailing commas via a lint rule that failed because the diff it produces it simply to large to be reviewed before git conflicts arise, let's try a more gradual approach and split the effort in separate PRs that apply only on a subset of the codebase at a time.