Disabled video start and end time test

Failed two times in a row:

https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/719/
https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/718/

@valera-rozuvan @polesye I'm going to merge this now; could you please add a story for re-enabling this test to the Blades backlog?