Make TestDatasetsAPI.test_list_elements independent of fixture's element IDs

Regenerating the tests fixture break that test. IDs must be reset manually in the fixture which is quite confusing.

l.1052 :

# id: ddc6a864-82d3-4f4d-9078-6fb547db7279
self.dataset.dataset_elements.create(element_id=self.vol.id, set="test")
# id: ebb0adee-91ff-4d21-bdfd-9dee5e61cd84
self.dataset.dataset_elements.create(element_id=self.page1.id, set="training")
# id: 1a55da30-0c90-4330-9533-02107aaaa93e
self.dataset.dataset_elements.create(element_id=self.page2.id, set="validation")
# id: b7b53865-24f3-4e09-b61c-de8bb1fcbd01
self.dataset.dataset_elements.create(element_id=self.page3.id, set="validation")