fix(tests): isolate mock class attrs leaking across tests (#12728)

tests/conftest.py

@@ -0,0 +1,21 @@
+from unittest import mock
+
+import pytest
+
+_MOCK_CLASSES = (
+    mock.Mock,
+    mock.MagicMock,
+    mock.AsyncMock,
+    mock.NonCallableMock,
+    mock.NonCallableMagicMock,
+)
+_MOCK_CLASS_BASELINE = {cls: frozenset(vars(cls)) for cls in _MOCK_CLASSES}
+
+
+@pytest.fixture(autouse=True)
+def _reset_mock_class_attributes():
+    """Drop attributes a test sets on the mock classes themselves (`c = mock.MagicMock; c.provider = ...`), so they cannot leak into other tests' instances."""
+    yield
+    for cls, baseline in _MOCK_CLASS_BASELINE.items():
+        for name in set(vars(cls)) - baseline:
+            delattr(cls, name)