Refactor enstaller tests and test basic indexed repositories facilities

This PR is a first step toward adding tests around fragile pieces of enstaller. Here, we test both the index repositories and the auto-update facility of enpkg, which are two areas that will see a lot of changes in a near future.

@sjagoe since you sounded interested in getting involved in build stuff, here is a small taste of it :) If you can't/don't have time, that's fine, I will assign it to someone else.