mpd: Test that "list album foo" responds correctly

Previously, test_list_album_with_artist_name would only test that the
command didn't fail. Now it also checks that the response is correct.
That is, that the response contains albums.

This makes the test detect the error reported in #817.
mpd: Fix list commands with 3 arguments (fixes #817)

List commands with 3 arguments should return albums, not artists.