Add an extra wait into test_opening_every_category_page_from_categories_menu. Fixes #559

My hope is that this will fix issue #559 

I'm running an adhoc at http://selenium.qa.mtv2.mozilla.com:8080/view/Marketplace/job/marketplace.prod.bsilverberg/1/. As this is intermittent, if it passes I will submit it a few more times.

@AndreiH r?