addTextStream(id, func) {
    const ContentType = shaka.util.ManifestParserUtils.ContentType;
    const stream = new shaka.test.ManifestGenerator.Stream(
        this, /* isPartial= */ false, id, ContentType.TEXT, 'und');
    if (func) {
      func(stream);
    }
    this.textStreams.push(stream.build_());
  }
Adds a text stream to the manifest.

@param {number} id
@param {function(!shaka.test.ManifestGenerator.Stream)=} func