Fix data leaks due to free-flying Journal instances

As discussed: with these changes all tests reliably & repeatably pass, both in eclipse and maven. Changes to the test superclass were minimal and while I'm not super-happy with the duplication of test setup/teardown logic it is symmetric, allows per-test isolation with minimum effort and works.
IMHO it would be less fragile to only create the directory & do cleanup in the superclass, and always have each test class create/open/close the per-test Journal it needs individually.