addPartialVariant(func) {
    const variant = new shaka.test.ManifestGenerator.Variant(
        this, /* isPartial= */ true);
    if (func) {
      func(variant);
    }
    this.variants.push(/** @type {shaka.extern.Variant} */ (
      jasmine.objectContaining(variant.build_())));
  }
Adds a new partial variant that, when used with jasmine, will only compare
the properties explicitly set on it.  Note that this will default to
having |null| audio and video streams.

@param {function(!shaka.test.ManifestGenerator.Variant)=} func