OZG-3936 move tests for linked resources to own test classes

...de/ozgcloud/alfa/bescheid/BescheidLinkedResourceTest.java

...de/ozgcloud/alfa/bescheid/BescheidModelAssemblerTest.java

...de/ozgcloud/alfa/bescheid/DocumentLinkedResourceTest.java

...de/ozgcloud/alfa/bescheid/DocumentModelAssemblerTest.java

...d/alfa/collaboration/CollaborationLinkedResourceTest.java

...d/alfa/collaboration/CollaborationModelAssemblerTest.java

...java/de/ozgcloud/alfa/common/LinkedResourceTestUtils.java

...gcloud/alfa/common/command/CommandLinkedResourceTest.java

...de/ozgcloud/alfa/historie/HistorieModelAssemblerTest.java

.../ozgcloud/alfa/kommentar/KommentarLinkedResourceTest.java

.../ozgcloud/alfa/kommentar/KommentarModelAssemblerTest.java