Add test libraries and coverage plugin

[vil1]

We want all the projects to use the same test library, scalaz-zio uses specs2.

The plugin already adds a test dependency to scalacheck in Compilation.scala, maybe we should move dependency-related settings in a separate file to avoid clutter in Compilation.scala.

We also need to add the scoverage plugin to the projects.
❓ Open question do we have a way to make sure from the plugin that the projects' CI configuration will run the coverage?

[harmeetsingh0013]

Why are we going with spects2 instead of scala-test?