Unit tests!

- [ ] login process (not sure how to do this - it's interactive oauth2)
- [x] session refreshing
- [ ] Track
- [x] loading track info
- [x] downloading tracks
- [x] track lyrics
- [ ] track metadata generation
- [x] loading album info
- [x] listing tracks from albums
- [x] loading playlist info
- [x] listing tracks from playlists
- [x] loading artists
- [x] listing albums from artists
- [x] loading cover arts
- [x] parsing URLs
- [x] searching
- [x] extracting client_id from Tidal Android `.apk`
- [ ] TidalMultiSession tests (what kind of?)
- [x] caching TidalObject creation
- [x] caching of tracks
- [x] caching of albums
- [x] caching of playlists
- [x] caching of artists

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_This issue has been automatically created by [todo-actions](https://github.com/apps/todo-actions) based on a TODO comment found in [tests/test_tidal_async.py:9](https://github.com/FUMR/tidal-async/blob/0a95543456aa14a3f2f55419fb0132c1633062be/tests/test_tidal_async.py#L9). It will automatically be closed when the TODO comment is removed from master._

Start with tests/test_tidal_async.py:9 and inspect the existing test structure and the unchecked items in the issue. Determine which remaining areas can be tested, including login, track metadata generation, and TidalMultiSession; done means adding the agreed unit coverage and removing the TODO comment so the automated issue closure can occur.

Written by the indexing model from the issue text.