Use === and avoid ==> in test suite

[konsumlamm]

Use === instead of ==, since it prints a counterexample on test failure and use Positive/NonEmpty instead of ==> to avoid discarded test cases.

[treeowl]

This is strictly cleaner. Using Positive will definitely reduce discarded values, since its Arbitrary instance takes the absolute value before verifying it has a positive result. NonEmptyList does the naive thing, so using it won't change performance, but it still seems nicer to use it.

[andrewthad]

Agreed about this being cleaner. It's strange that somehow, quickcheck has ended up doing the sensible thing for NonEmpty but the naive thing for NonEmptyList. But I'll leave that up to the quickcheck maintainers to improve one day.

Merging.