[DEV] Update unittest files for Windows and macOS / use defined static land_polygons / refactor unittest

[treee111]

This PR…

• updates the output files to compare against for malta and liechtenstein
• updates the static inpuf files for malta and liechtenstein
• uses a defined set of static land_polygons files and copy to common_download prior to running tests
• refactor test_generated_files.py
• clarify and unify comments of test_generated_files.py and test_downloader.py

Considerations and implementations

A defined set of land_polygon files need to be used for unittests because otherwise the output will differ.
The land_polygon files are not in the repo, they are in the parent folder unittest-files

Pull Request Checklist

• Reviewed the Contributing Guidelines
  • Especially the Pull-Requests section
• commits (and commit-messages) are understandable
• {...}