Add rollback last output functionality

Note that this does not include the transaction finalization which is
part of the rollback last output procedure in the opentransactions
wiki.
I've just renamed it to createFakeDecoratedTx.

One of the reasons is that the TstCreateCredits functions implicitly creates a series and thus can only be called once within a test (otherwise it will err with a series 0 already exists).

I'd certainly be able to work around that by creating another version of TstCreateCredits, but since I have no need for real credits, I didn't think it would be worth it.