Readd FFIWrapper.getwinerror for the mock to override

Author: A5rocks

src/trio/_core/_tests/test_windows.py

@@ -39,7 +39,8 @@
 def test_winerror(monkeypatch: pytest.MonkeyPatch) -> None:
     # this is unfortunately needed as the generated ffi is read-only
     class FFIWrapper:
-        pass
+        def getwinerror(self) -> None:
+            raise NotImplementedError("this is a fake implementation")
 
     mock = create_autospec(ffi.getwinerror)
     monkeypatch.setattr("trio._core._windows_cffi.ffi", FFIWrapper)