Fix test_browser_cell_data intermittent test failure

Fix the intermittent test_browser_cell_data.py failure. Wait for the throbber to appear first, and then wait for it to disappear (or the 40 seconds), before proceeding to verify the web page. Previously the test just verified the throbber wasn't displayed before continuing; sometimes the throbber doesn't appear right away after presing the URL button, so the test continued on before the throbber even appeared at all / before the page was actually loaded.