Clean up link_libraries in tests

There should be no need to explicitly link any targets but
KF5::BalooWidgets and Qt5::Test, indirect link libraries of BalooWidgets
are automatically included.

Exceptions are direct accesses of the KRatingWigets (WidgetsAddons) and
KConfig by the widget test, and usage of KFileMetaData properties in the
mergedata and extractor tests.