Update check_models_are_tested to deal with Windows path
# What does this PR do?

`TEST_FILES_WITH_NO_COMMON_TESTS` contains forward slash like `mt5/test_modeling_flax_mt5.py`.

The condition `if test_file in TEST_FILES_WITH_NO_COMMON_TESTS:` in `check_models_are_tested` would give failures to fix on Windows, like

```
camembert\test_modeling_camembert.py should define `all_model_classes` to apply common tests
```

This PR uses  `test_file.replace(os.sep, "/")` to make it work on Windows too 😄 
_The documentation is not available anymore as the PR was closed or merged._