Add test coverage

[celinval]

In #25 we're adding infrastructure to run rustc test suites to test StableMir. For each test case, we are running a set of tests to ensure the StableMIR works.

We should extend the tests that we are running for each test case to ensure we have a good test coverage for our APIs.

[celinval]

We have added a minimum coverage. We should now add coverage as we implement more APIs.