Author here, we wanted a clean room implementation and our own e2e test suite. There are some conformance tooling (like Apples calendar test suite) that we partially used (it's... very comprehensive), but otherwise we wanted to validate our library against existing implementations (manually, for the most part) and then write tests against our own implementation (for the interfaces, mostly to prevent regressions). We created a little CLI tool ("validav") that can spin up a mock server or expose the client interfaces to help with manual testing.

One niceish thing about WebDAV/CalDAV is it's pretty set in stone for now.