[JUnit] CollectionLayout#sortedBy should be effective based on actual runtime type

Double check whether for an entity that holds a relation of type Set<?> annotated with @CollectionLayout, the framework actually applies the specified comparator. (default is natural order).

My suspicion is that it only does this for compile time type SortedSet<?>. We might need to do a runtime type inspection, rather than rely on the meta-model, which does compile time inspection.