Improve test coverage

[mmontin]

Test coverage in cooked-validators is now tracked whenever somebody runs cabal test all. By taking a look at the coverage log produced by the command we notice that:

• current test coverage is decent and covers a huge chunk of the codebase (48% total) with a few modules with a 100% coverage rate.
• current test coverage is still not enough thought, with some modules with 0% coverage and some others with important features lacking coverage.

We need to bump these numbers. This would be unrealistic to aim for 100% coverage overall, but important features should be handled and this report should be used in that purpose.