test: extend case for timetable file delete

tests/test_scheduling_timetable.py

@@ -505,7 +505,15 @@ class TestSchedulingTimetable(unittest.TestCase):
         print(self._testMethodName)
 
         self.timetable.persist_timetable()
-        self.timetable.delete_timetable_file()
+
+        # Delete existing file
+        is_deleted = self.timetable.delete_timetable_file()
+        self.assertTrue(is_deleted)
+
+        # Delete non-existing file
+        is_deleted = self.timetable.delete_timetable_file()
+        self.assertTrue(is_deleted)
+
         self.assertFalse(self.timetable.timetable_file_exists())
 
     def test_merger_remove_timeslots_deleted_remotely(self):