fix: ensure that a test config is used for khal

tests/test_khal/helpers.py

@@ -1,12 +1,19 @@
-from tests.agenda_items import Valid
+from os.path import join
+from unittest.mock import patch
 
 from khalorg.khal.calendar import Calendar
 from khalorg.org.agenda_items import OrgAgendaItem
 
+from tests import static
+from tests.agenda_items import Valid
+from tests.helpers import get_module_path
+
 
 class Mixin:
 
-    def setUp(self) -> None:
+    @patch('khalorg.khal.calendar.find_configuration_file')
+    def setUp(self, patch) -> None:
+        patch.return_value = join(get_module_path(static), 'test_config_khal')
         args: list = Valid.get_args()
         self.agenda_item: OrgAgendaItem = OrgAgendaItem(*args)
         self.calendar = Calendar('test_calendar')