Jasmine test to verify that only one check for YT is made.

We test for the number of ajax calls made when there are 3
videos on the page.
Fix for test.

Must clear the state youtubeXhr variable before counting number
of ajax calls.
Addressing comments by Jean Michel for PR 867.

Forgot to include local variables in var statement.
Merge pull request #867 from edx/valera/video_single_yt_xhr_jasmine_test

Valera/video single yt xhr jasmine test