Move default storage tests into correct package

Move the tests for the default storage implementations into the correct
test package.
Clean up DefaultStoragesPersistenceTest

* The StorageContainer accepts all kinds of storage objects.
* The test uses DataObject instead of DefaultFeatureHolder for
referencing.
* Add some more complex tests using the DataObject class.
Remove printStackTrace() from some tests

Remove all printStackTrace() calls from all tests.
Add ReferenceCollectionStorage for collections of references

Add the ReferenceCollectionStorage which can store a collection of
referenced objects. It is required since JAXB must be able to retrieve a
collection type from a mapped property in order for collection
references to work correctly.
Update default storage implementations tests

* Add more Reference(Collection)Storage tests to the
DefaultStoragesPersistenceTest.
* Overhaul the DefaultStoragesTest in order to also test the
ReferenceCollectionStorage class.
Small storage implementation JAXB nnotation fixes

Fix some small style issues with the JAXB annotations of the default
storage implementations.