Fix use of GenderCache in ApiPageSet::processTitlesArray

Title::getPrefixedText was called before the GenderCache was set up,
which lazy loads the cache for each title,
resulting in one query per user title

Splitted the foreach to fill the cache at a better location

Added a test for ApiPageSet to test that the gender cache is filled

Bug: T200238
Change-Id: I7972dd1bf3731a92328caab20e70d7b9b82c1f7c