Consistency in labels of tests (underscore/dash)

[kinow]

The docs of conformance tests mention that we want to use underscores to separate strings in conformance tests labels. But we have some with dashes too.

+1 to being consistent. Maybe we could have a small GH Action written in Node.js to enforce it too (should take minutes to write… maybe a lot longer to test, but still worth doing it for future-proof I think).

Ref: #169 (comment)