Make test_existing_sop_instance_uid better match implementation

Author: carlosmartinez

tests/services/dicom/test_c_store.py

@@ -8,6 +8,7 @@
 from services.dicom import FAILURE, SUCCESS
 from services.dicom.c_store import CStore
 from services.dicom.image_compressor import ImageCompressor
+from services.storage import InstanceExistsError
 
 
 class TestCStore:
@@ -47,10 +48,11 @@ def test_no_patient_id_fails(self, mock_storage, mock_event):
         assert subject.call(mock_event) == FAILURE
 
     def test_existing_sop_instance_uid(self, mock_storage, mock_event):
-        mock_storage.instance_exists.return_value = True
+        mock_storage.store_instance.side_effect = InstanceExistsError("1.2.3.4.5.6")  # gitleaks:allow
         subject = CStore(mock_storage)
 
         assert subject.call(mock_event) == SUCCESS
+        mock_storage.store_instance.assert_called_once()
 
     def test_valid_event_is_stored(self, mock_storage, mock_event):
         mock_storage.instance_exists.return_value = False