fix: pass pytest xmodule VideoBlockIndexingTestCase

[Carlos-Muniz]

As part of #36, several tests have been discovered that are not passing in tutor, but passing in devstack. The following tests may not be run by tutor, or may be failing in tutor.

xmodule/tests/test_video.py::VideoBlockIndexingTestCase::

• test_video_with_multiple_transcripts_index_dictionary
• test_video_with_subs_and_transcript_index_dictionary
• test_video_with_youtube_subs_index_dictionary

For more information about the tests that passed, failed, skipped, or never ran, see the spreadsheet with all the tests.

[Carlos-Muniz]

This test(s) does pass in tutor. See #155 for more details.