It kind of sucks to make your jr devs do one of the hardest things first (writing tests for code that isn't written to be testable). To do this, you usually need to understand the testing tools inside and out, and you also need to understand the production code and all its dependencies very well. It's likely the person who wrote the code you want tests for doesn't even meet that bar--if they did, they would have written the code test first and there would be tests already.