Rethink “navigation-timing-2” test to avoid recurrent “latest-is-not-previous” errors

[tripu]

Every time there's a new version of the navigation timing 2 spec we have to manually update the metadata for the navigation-timing-2 test to avoid an error in the test suite (headers.dl.latest-is-not-previous) that breaks the builds — sometimes for days.

See #206, where I'm suggesting we disable that test until it's fixed.