TitleBlacklistTest singleton can now be destroyed
The test suite set $wgTitleBlacklistSources with a fixture source of
directories. Unfortunately when running tests from MediaWiki core the
TitleBlacklist has already been set with empty value and thus setting
the global is a noop.
That later causes a test to fail because the blacklist is emtpy.
Add TitleBlacklistTest::destroySingleton() so a test can reset the
singleton when changing $wgTitleBlacklistSources.
Since that is solely for testing, throw an exception unless we had