Fix for failing test test_the_displayed_message_for_incompatible_complete_themes

The page is opening with a hover card displayed, so added a step to clear hovercards.

I tested this locally and was able to reproduce the failure, and this code seems to have fixed it.

An example of the failure can be seen at http://selenium.qa.mtv2.mozilla.com:8080/view/AMO/job/amo.dev.saucelabs/1727/HTML_Report/