Restructure tests to allow for more than one test file

The current test structure has two issues:
1. Because the `Tests.hs` file is in the implicit `main` package, it's impossible for the student to add other test files. Complex solutions can be split into modules, and each module should be tested separately.
2. Advanced/optional tests can be maintained in a separate module that the student can run, while the test runner may only run the core tests.

Both of these problems can be solved by using [Hspec automatic spec discovery](https://hspec.github.io/hspec-discover.html). Probably will also fix https://github.com/exercism/haskell/issues/955.