Add missing Artists.get_all_artist_ids/0 test

test/music_library/artists_test.exs

@@ -14,4 +14,19 @@ defmodule MusicLibrary.ArtistsTest do
       assert expected == artist
     end
   end
+
+  describe "get_all_artist_ids/0" do
+    test "it returns unique artist IDs" do
+      marillion_record = record_with_artist("Marillion")
+      _another_marillion_record = record_with_artist("Marillion")
+      steven_wilson_record = record_with_artist("Steven Wilson")
+
+      [marillion] = marillion_record.artists
+      [steven_wilson] = steven_wilson_record.artists
+
+      expected = MapSet.new([marillion.musicbrainz_id, steven_wilson.musicbrainz_id])
+
+      assert expected == Artists.get_all_artist_ids()
+    end
+  end
 end