Restructure tests to allow for more than one test file

[asarkar]

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. Probably will also fix #955.