Follows-up 6f8dc27ca2 and dbd11e04aa. You'd expect a bug in preloading
to fallback to fetching it on-demand but due to a title format mismatch
the preload method did use the same title format for the cache key, but
not the same title format for discovering the results. As such, it set
the right cache key to an empty array.
* Make relevant methods testable and mockable.
* Add regression test.
* Change call to array_interect_key to use the same format as
fetchTitleInfo(): Normalised title keys from Title::getPrefixedText.
Previously, the intersect used the declared titles from getPages() which
are not localised - causing an empty array to be returned from the
intersect on wikis where the namespace name is localised.
Bug: T145673
Change-Id: Ibe788157724d73c727b9e2127b6828db32ca9420
dbe592df6d