explicitly stop mock_backend to be unavailable

tests/test_plugin.py

@@ -134,8 +134,9 @@ def test_instrument_products(dispatcher_live_fixture, mock_backend):
                          'table': 'table_query'}
 
 
-def test_instrument_backend_unavailable(dispatcher_live_fixture):
+def test_instrument_backend_unavailable(dispatcher_live_fixture, mock_backend):
     # current behaviour is to have instrument with no products, could be changed in the future
+    mock_backend.stop()
     server = dispatcher_live_fixture
     logger.info("constructed server: %s", server)