Added explicit wait to failing JS test

Saw this failure in master:
https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/250/SHARD=1,TEST_SUITE=unit/testReport/junit/JavaScript/firefox/Transcripts_Editor_Test_synchronization_Test_Basic_to_Advanced_synchronization__Correct_data/

This test has been passing consistently for a long time, and it continues to pass for me locally.  I'm guessing this is a race condition between the test and Backbone.js, so I added an explicit wait for the default value to go away.

Not super-elegant, but safer.

@jzoldak