Write tests for services

Because the test coverage for services currently looks quite pathetic. 

- [ ] `services/info/album`
- [ ] `services/info/artist`
- [ ] `services/info/song`
- [x] `services/download`
- [x] `services/ls`
- [x] `services/playback`
- [ ] `services/socket`
- [x] `services/youtube`