test coverage CI and codecov.io is officially working!

[kyle-messier]

@dzilber @eva0marques @sigmafelix @dawranadeep @MAKassien @mitchellmanware @larapclark @franksday @Sanisha003

Hi Y'all! I'm happy to say that the test-coverage action on continuous integration, and more importantly, the full code coverage report integration with codecov.io is working! We can start to see whether every line of code in R/ is "covered" by a test in tests/testhat/.

I was able to write a very simple test that covers some lines of code in the generic_base_learner R function that @dzilber wrote, which is reflected in the codecov badge on the README

Since, we have the main repo passing all tests, we are now implementing an additional repo rule: For a merge to be approved, the test-coverage action must pass.