Update test_download.py

[deldesir]

Description

This PR includes changes to the test script to ensure that the --write-thumbnail option in xklb is properly tested. The test now asserts the existence of the thumbnail file after the download process.

Motivation and Context

This change is required to verify that the --write-thumbnail option works as expected. Ensuring the thumbnail file is downloaded correctly helps finding it by third party tool (iiab/calibre-web for example)

How has this been tested?

The changes were tested by running the updated test script in an environment with internet access. The test downloads a video from YouTube and checks for the existence of the thumbnail file in the specified directory. The test verifies the following:

• The video and subtitles are downloaded correctly.
• The specified thumbnail file is created and exists in the storage directory.

Testing environment:

• Operating System: Ubuntu 24.10
• Python Version: 3.12.3
• xklb Version: 2.8.050

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[chapmanjacobd]

Also, if a PR is related to an existing issue don't worry about filling out the PR description. Just write a good PR title and you can replace the whole description with a link to the related issue/comment

Thanks !