DatabaseShutdownRollbackTest: Fix false unit test failure

- Symptom would be JUnit failing with "Test class should have exactly
one public constructor" about the Leak and Thing member classes.
- Reason is that JUnit will consider member classes of test classes as
test classes themselves, but they were not meant as such in this test.
They are merely helpers for their parent class.
- I did not notice this when committing the tests because it only
happens with the ant test runner, not with the Eclipse one. Sorry.
- Notice: The tests will still fail due to
test_WillAlwaysFail(DatabaseShutdownRollbackTest.java:66) failing with
a regular test failure (AssertionError). This is intentional breakage
to force developers to fix the following issue before deploying a new
build: https://bugs.freenetproject.org/view.php?id=6247. It is
specified at the class level JavaDoc of the affected test already.
DatabaseShutdownRollbackTest: Disable always failing test because the
underlying issue was postponed

- The test was specifically added to enforce fixing of
https://bugs.freenetproject.org/view.php?id=6247 before deploying the
next build. (This is also specified at its class-level JavaDoc)
- It was decided to postpone the issue to WOT 0.4.2, so I am disabling
the always failing test now.
Merge branch 'master' of https://github.com/freenet/plugin-WoT-staging