I've had a local LLM spending weeks trying to write tests. then debug those tests. then write antipatterns and patterns for those tests.
It's amusing. It's not terrible, but tests arn't going to save you from a malicious tester.
> I would expect a mature code base like rsync to have a lot of unit tests and integration tests
You might be surprised. C applications which interact heavily with the system - like rsync - can be tricky to test comprehensively, as it's nontrivial to inject faults into system calls. If the application is architected to support this kind of testing, or uses a HAL, that may make matters easier - but an older codebase like rsync probably isn't.
I have been experimenting with both aforementioned styles with interesting results.