Hackage4Nix.hs: relax the patters that recognizes all-packages.nix as a 'bad' package
Hackage4Nix.hs: preserve state of the 'doCheck' field
Distribution/NixOS/Derivation/Cabal.hs: render the 'doCheck' field only when it's set to 'false'

'true' is the default anyway.
Distribution/NixOS/Derivation/Cabal.hs: add a field 'testDepends'

Test dependencies are buildInputs required only for building (and
running) the test suite.
Cabal2Nix.hs: added a '--no-check' option to set 'doCheck=false' in the generated expression
Distribution/NixOS/Derivation/Cabal.hs: initialize 'doCheck=true' by default