change the test harness' session initialization s.t. we can configure it for per-test needs. Add tests for moving completed torrents when downloadDir is a subdirectory of incompleteDir. (Hi, KyleK)

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13916 f4695dd4-2c0a-0410-b89c-da849a56a58e