Adding wait for test_marketplace_search_and_install_app.py fail

this addresses http://qa-selenium.mv.mozilla.com:8080/job/b2g.unagi.mozril.gaia.v1-train.ui.marketplace/lastCompletedBuild/testReport/%28root%29/TestSearchMarketplaceAndInstallApp/test_search_and_install_app/
fail that I could reproduce locally. I think that <code>switch_to_homescreen_frame()</code> was happening to fast

After adding this wait I could not reproduce the fail anymore and got 10/10 passes