Fix flakey js caption test

I'm seeing some flakiness in the JS video caption tests:

https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/189/SHARD=1,TEST_SUITE=unit/testReport/junit/JavaScript/firefox/VideoCaption_scrollCaption_when_not_frozen_when_there_is_no_current_caption__does_not_scroll_the_caption/

I think that the test is resetting the `scrollTo` mock before it resets the video player.  The video player can therefore still call `scrollTo` after it gets reset, which messes up tests.

@jzoldak