Add tests for sequencing HLists of Kleislis

The one for Kleislis of ValidationNels results in an implicit not found
error and is commented out.

@larsrh (or anyone else) do you know of a way to convince the compiler to pick up the sequencer for a Kleisli of a ValidationNel? (See commented out test)

EDIT: never mind. My type alias just needed to be covariant. I originally added these tests just to point out this issue, but even though there isn't an issue, I guess there's no harm in further test coverage.