In #3170 we introduced a cycle check to the solver. This check is necessary to
reject cycling solutions (which would previously have resulted in an internal
error when we verify the install plan). However, this by itself is not
sufficient. If we have a cycle through setup dependencies, the solver loops
because it starts building an infinite tree. This is explained in detail in

In this commit we just add some unit tests that provide a minimal example that
exposes the bug.