NodeStarter.globalTestInit(): Allow specification of a RandomSource

- This will allow unit tests to specify a DummyRandomSource so their
Nodes won't block for a long time waiting for Entropy.
- I am putting this into plugin-fcp-rewrite because I am adapting the
WOT unit tests to be able to use them to test plugin-fcp-rewrite. They
will create a full node, and the startup time has already started to
annoy me :)