Fix some svtplay_dl.output related fixmes

This set of changes will make svtplay_dl.output.progress() wrap progressbar() when total size is known, and keep writing the more "basic" progress indication when it's not. It also tidies up the bar generation of progressbar(), and finally, introduces some unit tests for progress().

I may add some more tests for progress() in the near future. (Verifying expected output etc)