QuickCheck 2.7 compatibility

QuickCheck 2.7 compatibility

This makes our test compile with out errors with QuickCheck 2.7.
Warnings about the deprecation of printTestCase remain when using 2.7.

This change is backwards-compatible with all older versions of QuickCheck
that we support.

In 2.7, Property is no longer a monad, but remains a `Gen Prop` inside,
so that we only have to use combinations of `property` and `return`
to become compatible.

See
  https://hackage.haskell.org/package/QuickCheck-2.7.6/changelog

Further, in QuickCheck 2.7, Positive/NonZero/NonNegative are no longer
instances of `Integral` (NonNegative could likely still be one, see
https://github.com/nick8325/quickcheck/issues/31).
Consequently we cannot create them using `fromIntegral` any more,
and switch to `fromEnum` instead, which also is backwards-compatible.

Signed-off-by: Niklas Hambuechen <niklash@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>