Introduce a real testing suite

[thomasvergne]

There is a real need to implement a fully-working testing suite for Plume. This may improve the following points:

• Compiler development: Bugs would be easily fixable or at least easily findable.
• General stability: Early compiler bugs would be caught by the testing suite and would avoid binary building issues with the GitHub action.

In other words, tests would improve the development quality.

Implementation

Unit testing could be an option, but it is not viable as this would require huge pieces of hand-made AST. So we're rather thinking of a hybrid system:

• Parser implementation would be verified by unit testing, as this is just parsing validation on raw code
• Other modules would depend on this parser verification and should use the parser to generate ASTs from raw code instead of manually managing and writing ASTs

Haskell libraries

• HSpec - Library used to verify parser implementation (combined with Megaparsec extension)