Most of the logic in those classes is tested via Clojure-based tests in the test suite, often via generative tests that create 1000s of test examples. Feel free to check out the bug tracker (
http://dev.clojure.org/jira/browse/CLJ) or changelog (
https://github.com/clojure/clojure/blob/master/changes.md) for possible areas more likely to see issues.
We're not interested in refactoring or changes just for the sake of changing things. Regarding JUnit, we don't have (or want) JUnit-based tests in the test suite. We prefer generative tests with test.check (or check-based spec tests) and secondarily example-based tests with clojure.test.
I think it would be a great exercise to pick a piece of core Clojure functionality and write generative tests for it, particularly if it's something not already covered. Adding robust generative tests is something worth considering. Note that there are generative tests written using both test.generative (generally older ones) and test.check (newer ones) in the suite and I would strongly prefer the latter at this point.
I guess one caveat is that any new generative tests should also consider leveraging the new clojure.spec.check functionality for checking functions, which implies the existence of specs for core functions, which is something in progress but not yet released. So... that part might be a little tricky right now.
Of course, if you write tests however and discover a problem, that's certainly something we'd want to know about. :)