Review: I think this is a safer way to do it. In the end, the error says "can't remove the root of a hierarchy", so it makes sense to exactly guard against this, and also it then only uses the general JUnit Platform API and doesn't rely on ArchUnit specifics (and `AbstractArchUnitTestDescriptor` is more an implementation detail, it's more by accident that the rule descriptors, etc., inherit from this, but the engine descriptor doesn't).