tests: Fix check to be compatible with .lock

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tests/test_updater_top_level_update.py

@@ -135,8 +135,7 @@ def test_cached_root_missing_without_bootstrap(self) -> None:
             self._run_refresh(skip_bootstrap=True)
 
         # Metadata dir is empty
-        with self.assertRaises(FileNotFoundError):
-            os.listdir(self.metadata_dir)
+        self._assert_files_exist([])
 
     def test_trusted_root_expired(self) -> None:
         # Create an expired root version