Test and benchmark LanguageController::languagesForUrl()

LanguageController's initialize() and cleanup() member functions were
called once each by Core and didn't reset LanguageController's state.
This commit expands the definition of initialize() to make these two
member functions work for the added tests. Most of the added code
belongs in cleanup(), but moving it there seems a bit risky: what if
this data is used at KDevelop exit?