Make tab nav page object more robust

There was a failure in the bok-choy test suite this morning: https://jenkins.testeng.edx.org/job/edx-bok-choy-auto-master/39/console

From the traceback, it looks like the top-level tabs hadn't loaded before the test tried to click "Courseware".  This fix adds a promise that at least one tab name is loaded before attempting to go to a particular tab.

@jzoldak