Prepare for revdep testing

[cgwalters]

Just a placeholder issue to track the goal: Let's add a job (perhaps opt-in) that triggers running tests from this repo on bootc-dev/bootc CI - pattern looks like e.g. https://github.com/composefs/composefs-rs/tree/main/bootc

[jlebon]

Related: #74 where I suspect revdep testing would be even more valuable just because the composefs backend is fresher and churns more.