I really like that you come at it fairly 'bare bones'... as in, no Rspec / FactoryBot / Fixtures / Cucumber / Faker / shoulda-matchers /guard etc etc. I recall when learning for the first time, it was just so much to take in for testinig. Your PORO approach makes it so simple and not as intimidating.

+1 for minitest & fixtures over rspec, factorybot etc This is nice.

What is the test suite that Chris is using here? Is there a book that shows how to use this test suite?

It's just minitest. https://guides.rubyonrails.org/testing.html