(fix): load_extra_path_config: relative path not converted to a full path

[bigcat88]

No breaking changes, just a small fix + more tests.

1. Added conversion of base_path to absolute path if it is relative, for correct detection of duplicate paths (and the add_model_folder_path function accepts full_folder_path: str, so it is logical to do so)
2. Added 3 tests for all cases of extra_model_paths.yaml: when base_path is relative, when it is absolute and when it is not there at all.
3. Fixed old tests "load_extra_path_config", added OS detection, because the path C:/Users/TestUser/AppData/Roaming/ on Linux is considered relative.

[bigcat88]

@robinjhuang I see from the gitlog that you wrote tests in this file, can you review the PR?

p.s.: I don't know why monkeypatch.setattr is used instead of the usual file writing to the temp directory (which will make the tests more readable in my opinion) - if necessary, I can rewrite the tests for writing files to the temp directory.