Test osbuild module schemas with osbuild CLI

[achilleas-k]

Add unit tests for each osbuild module that generates a minimal manifest and checks it against osbuild for validation to verify that our schemas are correct.

We could even probably fuzz this kind of test. If our schemas, serialisers, and validators in this repo are correct, creating a stage should either return an error (or panic) if the values are invalid or should succeed against osbuild --inspect.

One thing that might be useful to test (in general) is that our sources/inputs/stages render to the expected json that osbuild needs. Having such tests would allow us to refactor how stages generate json and we would have a corpus of valid/expected json to double check against. And arguable it's the most important aspect of the stages/inputs/sources - that they render to osbuild json. But just an idle though/converstaion topic, not something that need to change in this PR of course :)

Originally posted by @mvo5 in #448 (comment)