Change wait logic for expanded section to use class instead of visibility

This addresses the failure as seen at [1]. I guess the markup changed so that now the element that used to be visible is no longer visible, which was causing the wait to time out. Tbh, I'm surprised it worked before, but evidently it did.

[1] http://qa-selenium.mv.mozilla.com:8080/job/amo.dev.mac/198/testReport/junit/tests.desktop.test_details_page/TestDetails/test_that_the_development_channel_expands/