pkgs/top-level/haskell-defaults.nix: cosmetic
haskell-packages.nix: add 'disableTests' convenience function
haskell-defaults.nix: don't pass inconsistent Cabal versions to cabal2nix
jailbreak-cabal: build jailbreak-cabal with a version of Cabal that has its unit tests disabled

Cabal's unit test suite depends on testFrameworkQuickcheck2, which
depends on jailbreakCabal, which depends on Cabal: *BOOM*