cargo-kani self-test

[tedinski]

We currently run a few smoke tests in CI on various platforms to check whether things behave correctly. We should capture this into a behavior that can be run with one simple command.

This will simplify CI (just run self-test) and let customers run a quick self-test on their own local install, to validate everything seems installed and working correctly.

[tedinski]

We run some ad-hoc smoke tests that could become the self-test procedure here:

https://github.com/model-checking/kani/blob/main/.github/workflows/kani.yml#L126

1. The idempotency one wouldn't be part of self-test.
2. We should add tests for the kani binary, too.