The 'subpage' option in parserTests is supposed to turn on subpage support.
(which is a wiki config thing?)
But parsoid always enables subpage support; that is, /foo will always
link to [[Parent/foo]], not an article titled '/foo'.
We should make that a wiki config option, fetch it via the API, and only resolve subpages if it is turned on. [We could also make the 'subpage' option in parserTests work properly then.]