Are you talking full-on unit testing? If so, good luck. The entire codebase is a mess.
That said, if whatever you're doing involves changes to the source (including documentation, refactoring, or whatever), I encourage you guys to shoot us a PR with any changes you make. And if you guys need any help navigating the minefields hidden through the source, feel free to hit up our IRC channel (#srb2fun on irc.esper.net) and someone should be able to assist you.
Honestly, if you guys want specific requests for things to test/document, I'd love to see the software renderer dissected more. (It's somewhat contained within the r_*.c files, for a place to look.) It's probably one of the better organized and documented (I use those terms lightly) parts of the codebase due to not being messed with by too many people, and there's some pretty cool stuff going on back there if you've never dissected the Doom renderer before.