Disable video caption test seen to fail in master.

@jzoldak 
@valera-rozuvan 

Failed here: https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/SHARD=1,TEST_SUITE=unit/239/testReport/junit/JavaScript/firefox/VideoCaption_scrollCaption_when_frozen__does_not_scroll_the_caption/