test_base: add test for non-numeric SubmodelElementList Key values

A duplicate test is replaced by this test.

test/model/test_base.py

@@ -981,9 +981,15 @@ def get_identifiable(self, identifier: Identifier) -> Identifiable:
  self.assertEqual("'Referable with id_short prop_false not found in "
  "SubmodelElementCollection[urn:x-test:submodel / list[0]]'", str(cm_8.exception))
 
+ ref9 = model.ModelReference((model.Key(model.KeyTypes.SUBMODEL, "urn:x-test:submodel"),
+ model.Key(model.KeyTypes.SUBMODEL_ELEMENT_COLLECTION, "list"),
+ model.Key(model.KeyTypes.SUBMODEL_ELEMENT_COLLECTION, "collection")),
+ model.SubmodelElementCollection)
+
  with self.assertRaises(ValueError) as cm_9:
- ref9 = model.ModelReference((), model.Submodel)
- self.assertEqual('A reference must have at least one key!', str(cm_9.exception))
+ ref9.resolve(DummyObjectProvider())
+ self.assertEqual("Cannot resolve 'collection' at SubmodelElementList[urn:x-test:submodel / list], "
+ "because it is not a numeric index!", str(cm_9.exception))
 
  def test_get_identifier(self) -> None:
  ref = model.ModelReference((model.Key(model.KeyTypes.SUBMODEL, "urn:x-test:x"),), model.Submodel)