Looks like it's already fixed

Initial code is convex :: Num a => [(a,a)] -> Bool, but the kata is not solvable without at least Eq a or Ord a, so the initial code should have that added too.

Zero-length edges would merit a mention in the description; they would be somewhat counterintuitive.

Fixed testing with adjacent parallel edges would be nice though.

Test headers seem to imply there's only one test being run. Actually, it's testing the usual 100. I'd mention that in the header; it prevents people ( me, if I hadn't looked into it ) from complaining.

You should add more edge cases to the tests, such as adjacent parallel edges and zero-length edges.