Make sure that ctrl-c works properly with GHC 7.8.* and QuickCheck-2.6

The `interrupted` flag does not work with GHC 7.8.* (see
https://github.com/nick8325/quickcheck/commit/57bfa874b3d9733afd32da98f83cadfa2d9fffed).
This is why we now rely on string comparison instead (the way we already
do it for QuickCheck < 2.6).