Improve tests for stores

[phanan]

Current tests for stores rely on a data blob. They should be rewritten to use factoria models instead.

• [ ] album.js
• [ ] artist.js
• [ ] equalizer.js
• [ ] favorite.js
• [ ] index.js
• [ ] playlist.js
• [x] preference.js
• [ ] queue.js
• [ ] recentlyPlayed.js
• [ ] setting.js
• [ ] shared.js
• [ ] song.js
• [ ] user.js