Add tests for prototype utilities

[pmeier]

Perhaps we should add tests for sequence_to_str, so as to ensure that our error message are properly formatted ?

Also nit but if we're going to have such a helper, we could handle the case a, and b -> a and b (i.e. remove the comma if len == 2

Originally posted by @NicolasHug in #5187 (comment)

cc @bjuncek