bug 1461350: Add tests for lang fixes, legacy 404s
bug 1461350: Add tests for lang fixes, legacy 404s

Add tests for MDN-specific locale fixes, such as cn → zh-CN, and for
legacy URLs that should 404, such as index.php. These were previously
tested only in headless tests, but are part of the URL contract.

SphinxKnight/kuma,SphinxKnight/kuma,escattone/kuma,Elchi3/kuma,SphinxKnight/kuma,jwhitlock/kuma,a2sheppy/kuma,escattone/kuma,SphinxKnight/kuma,mozilla/kuma,Elchi3/kuma,mozilla/kuma,jwhitlock/kuma,a2sheppy/kuma,jwhitlock/kuma,SphinxKnight/kuma,Elchi3/kuma,a2sheppy/kuma,mozilla/kuma,a2sheppy/kuma,jwhitlock/kuma,a2sheppy/kuma,SphinxKnight/kuma,jwhitlock/kuma,escattone/kuma,mozilla/kuma,mozilla/kuma,Elchi3/kuma,Elchi3/kuma