Video Block Unit Test | Move test_video.py from 'edx-platform' to 'xblocks-contrib'

[farhan]

Move https://github.com/openedx/openedx-platform/blob/farhan/enable-video-block-4/xmodule/tests/test_video.py
to xblocks-contrib repo.

WIP PR: openedx/xblocks-contrib#144

[farhan]

As per Irtaza, following file will help:

xmodule/tests/__init__.py