fix tests

see comments on commit e2b4b1b06fd3abf41bc20aaea06d2f0403b0e785

src/MusicalScore/ScoreIO/VoiceGenerator.ts

@@ -45,6 +45,7 @@ export class VoiceGenerator {
     } else {
       this.voice = new Voice(this.instrument, voiceId);
     }
+    this.instrument.Voices.push(this.voice); // apparently necessary for cursor.next(), for "cursor with hidden instrument" test
     this.staff.Voices.push(this.voice);
     this.lyricsReader = new LyricsReader(this.musicSheet);
     this.articulationReader = new ArticulationReader();