Changed the wait for page to load

This fixes http://qa-selenium.mv.mozilla.com:8080/view/Marketplace/job/marketplace.dev/lastCompletedBuild/testReport/tests.desktop.consumer_pages.test_consumers_page/TestConsumerPage/test_opening_every_category_page_from_categories_section/
Method <code>is_element_not_visible</code> is there, so why not use it?
i don't know if that wait was correct
Seems to fix the test for me, tested locally