test: Fix spaces and disable messages from pylint.

Unfortunately, we have no control over the names or number of methods that
the standard python library employs.
Add support for debugging information.
Rework the download_videos function.
Only print or pass the video ids for youtube-dl.

We don't need to pass entire URLs to youtube-dl, as it gladly accepts only
the video ids.
Add some very dirty code that's just an skeleton of what I want.

The *very* basic functionality is there, but we have to improve a lot in
terms of cleanliness.
modularization: Move first functions to a separate file.

With this, we may, perhaps, achieve some reusability between projects and,
also, ease implementation of unit tests.