QA: wait for element to appear

prevent false failures like
https://integration.wikimedia.org/ci/view/BrowserTests/view/-All/job/browsertests-MultimediaViewer-en.wikipedia.beta.wmflabs.org-linux-firefox-sauce/lastCompletedBuild/testReport/(root)/Download%20menu/The_medium_download_option_has_the_correct_information/

Change-Id: I122a82a5ec802c0c0eec90b55757153a2ab58ef7