fix webdav tests and then add some (#6189)

tests/utils/asserts.py

@@ -1,7 +1,12 @@
-from typing import Any
+from typing import Any, Dict
 from unittest.mock import ANY, Mock
 
 
+def issubset(subset: Dict, superset: Dict) -> bool:
+    assert {**superset, **subset} == superset
+    return True
+
+
 def called_once_with_subset(m: Mock, *args: Any, **kwargs: Any) -> bool:
     m.assert_called_once()
     m_args, m_kwargs = m.call_args