Fix tests if search engine is changed

test_ephy_web_view_normalize_or_autosearch(): don't assume the default
search engine has never been changed.

https://bugzilla.gnome.org/show_bug.cgi?id=721348
Improve test_ephy_web_view_normalize_or_autosearch

Run the test using a search engine other than DuckDuckGo, to make sure
changing the search engine works as expected.

https://bugzilla.gnome.org/show_bug.cgi?id=721348