soundcloud: impoved soundcloud helper to include descriptions for tracks

tests: actually added soundcloud test to test_all :)

Soundcloud has a track 'description' field that isn't in their [API documentation](https://developers.soundcloud.com/docs/api/reference#tracks).  Seemed fitting once I stumbled upon it to use it in the helper.

Also seemed fitting to actually add the soundcloud tests to test_all... :)