FoldableTests are not testing sequence_/traverse_

[johnynek]

see: typelevel/cats-collections@4812b86#diff-1f70710e148c78ab48fdd3687d6ecd0bR201

I had to add that to get full test coverage.

We could have a law that: fa.sequence_ == fa.foldMap(identity) where we construct the natural Monoid[G[Unit]] for an Applicative.

More generally, I think we should have a practice that each instance matches an instance with only the abstract methods proxied through. This guarantees that all the other methods are only override for efficiency, not to change any semantics. If we made sure to test this way we could simplify our tests since we only need to write laws on the abstract methods, and then the additional law that we match the proxied instance.

[peterneyens]

we should have a practice that each instance matches an instance with only the abstract methods proxied through

See also #1684.

I tried quite a while ago to generate these sort of tests with scalameta, but I stopped I think when I had to deal with inheritance (for example Apply where we also need Functor#map).