I don't think I've met a codebase that had tests that would work with the level of refactoring that would need to be done. I mean let's be honest here, if it was well architected, refactoring wouldn’t be like the comic. It’s probably a mess of entangled concerns and bad inheritance.

For that matter the code most in need of refactoring probably has shit tests and that just assertNotNull on the happy path to appease sonar and be done with it.