AbstractFullNodeTest: Use faster RNG, print out seed

The previously used globalTestInit() uses Yarror, which waits for
entropy during startup, and didn't allow specification of a seed.
Instead, we use the DummyRandomSource provided by the base class
AbstractJUnit4BaseTest.
AbstractFullNodeTest: Adapt to new signature of
NodeStarter.globalTestInit()

- It was changed to consume a File baseDirectory instead of String
testName in fred plugin-fcp-rewrite commit
9fa77b61745874d1e2fbbcd25538c5c683e58a82. The String was previously
actually used as a directory name.
- It is safe to use the same directory as what is used for
createTestNode() afterwards. See the JavaDoc of globalTestInit() /
createTestNode() which I've added in the commits up to the one mentioned
above.
AbstractFullNode: Implement auto deletion of node directory

- Uses JUnit4's class TemporaryFolder. Nice that it has this! The class
will automatically delete the folder. Also it will fail if it exists
already, I have manually validated that for JUnit 4.11 which seems to be
the most recent and also in my package manager. Therefore, the asserts
which validate that it is deleted and doesn't exist can be removed since
it is JUnit's job to work properly now.
- Also adds TODO to the old unit tests which create a database file to
use this.