Add kdf_iterations config set option and test

src/tests/test_cli_config_set_extra.py

@@ -15,6 +15,7 @@ runner = CliRunner()
         ("clipboard_clear_delay", "10", "set_clipboard_clear_delay", 10),
         ("additional_backup_path", "", "set_additional_backup_path", None),
         ("backup_interval", "5", "set_backup_interval", 5.0),
+        ("kdf_iterations", "123", "set_kdf_iterations", 123),
         (
             "relays",
             "wss://a.com, wss://b.com",