Unit-Testing Frameworks: Catch2

It isn't extremely complicated to write a tiny unit-testing driver. It might not be pretty, but believe it or not, professional developers actually do like to reinvent the wheel (theirs will be fancier, rounder, and faster than the legacy one). Don't fall into this trap: we'll create so much boilerplate that it could become a separate project. Introducing a popular unit-test framework to our solution aligns it to a standard that transcends projects and companies and will get us free updates and extensions for cheap. We can't lose.