Re-enable the tests

[gelisam]

In #173, I disabled the tests. This was meant to be a temporary solution: let's enable them again.

Last time I worked on hawk (#155), I got stuck because the tests were green on my machine but not on Travis. Years layer, the situation has worsened: the tests seem to have bit-rotted, and the tests don't pass on my machine either anymore. There were too many failures so simply fix them all, and I am expecting some contributions soon, so I don't want the failing tests to be an obstacle to those contributions. So I have disabled the tests for now.

Since there are so many failures, my plan to make re-enabling the tests more manageable is to split the work into several smaller issues. The scope of this issue is:

• figure out which modules have failing tests
• for each failing module, open an issue to re-enable its tests
• re-enable the modules which don't have failing tests

[gelisam]

I found three doctests fail in System.Console.Hawk.Args.Parse (#198), ten doctests fail in Control.Monad.Trans.OptionParser (#199), twelve doctests fails in System.Console.Hawk.PreludeTests (#200), and one failing test in System.Console.Hawk.Test (#201).

[gelisam]

The tests pass with stack test and cabal test (after cabal sandbox init and cabal install --enable-tests), but not with cabal new-test. With #186 still unresolved, I guess that shouldn't be surprising!