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.

Start by reading the current Tests.hs arrangement and the Hspec automatic spec discovery documentation linked in the issue. Restructure the test setup so students can add multiple test modules, while optional tests can remain separate from the core test runner; also check whether this addresses issue 955.

Written by the indexing model from the issue text.