function verifyHelper(vttCues, shakaCues) {
    mockTrack.addCue.calls.reset();
    displayer.append(shakaCues);
    const result = mockTrack.addCue.calls.allArgs().reduce(
        shaka.util.Functional.collapseArrays, []);
    expect(result).toEqual(vttCues.map((c) => jasmine.objectContaining(c)));
  }
Verifies that vttCues are converted to shakaCues and appended.
@param {!Array} vttCues
@param {!Array<!shaka.text.Cue>} shakaCues