Added wait for course updates dialog to disappear

Response to this failure: https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/449/SHARD=2,TEST_SUITE=cms-acceptance/

From the screenshot, it looks like the dialog didn't finish closing before the test tried to edit the fields on the page.  I added a wait for the form to disappear to make this more robust.

Neither the test nor the code had been touched in a long time, so I'm not especially worried about this test.

@jzoldak