Use cabal's test-suite section

[Fuuzetsu]

It'd be great if the package could use cabal-supported tests: the existing way is quite inconvenient to run and cabal is not informed about the test dependencies that we need (json, QuickCheck, binarydefer, cereal). It also makes it very difficult to test the project inside a cabal sandbox.

[ndmitchell]

Agreed, that would be useful. I haven't done any significant work on the tests in derive in a long time, I think when I wrote them we didn't even have the test mechanism, which is why it doesn't currently use that.

[ndmitchell]

I have modified the Travis buildbot to run the generator test, which does work - running travis.sh is probably sufficient for the moment, but a proper cabal test would be better (although I'm not sure if calling GHC itself during the test suite is encouraged).