Replace NEST_AGE_DEFAULT by "unknown"

observations/tests/test_management_commands_auto_qa_checks.py

@@ -181,7 +181,7 @@ def test_flag_nests_with_uncertain_nest_age(self, mock_get_logger):
         TurtleNestEncounter.objects.create(
             site=self.area,
             status=Encounter.STATUS_IMPORTED,
-            nest_age=NEST_AGE_DEFAULT,
+            nest_age="unknown",
             species=TURTLE_SPECIES,
             when=datetime.now(pytz.utc),
             where=Point(0.0, 0.0)