Test suite improvements

[andreabolognani]

The current test suite is lacking in a number of ways.

Some of the packages (repo, rpm) use gomega only partially or don't use it at all.

Some of the packages (api, ldd, reducer) have no associated tests.

One of the tests (pkg/repo/repo_test.go) tries to open the non-existing repo.yaml file, but doesn't consider the failure to be fatal and happily reports a success status.

We also have a large number of references to obsolete content (Fedora 32). We should get those updated and document the process so that we can keep them reasonably fresh over time.

I'd be willing to work on some of these items, but I will probably need guidance because a lot of the inner workings of bazeldnf are still quite fuzzy in my head.