Add a check for beta users in the video_info API

Discovered a case I missed on the original version of this. See
https://github.com/edx/edx-platform/pull/5798

@gwprice @ormsbee 

JIRA: MA-76
The case with role=None is redundant with test_non_mobile_enrollments. I think the best thing to do would be to get rid of that test function and use its name for this one.