Add explicit wait to JS transcript tests

Should fix this issue:

https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/SHARD=1,TEST_SUITE=unit/266/testReport/junit/JavaScript/firefox/Transcripts_Editor_Test_synchronization_Test_Basic_to_Advanced_synchronization__Correct_data/

This is the same underlying cause as #1487 

@jzoldak @polesye