Bolster test_partner_images_pager_two with a wait

This addresses the intermittent failure as seen at [1].

[1] http://qa-selenium.mv.mozilla.com:8080/job/mozilla.com.prod.win2008.chrome.saucelabs/lastCompletedBuild/testReport/tests.test_partners/TestPartners/test_partner_images_pager_two/history/?