Improve PingPongTransactionChain coverage

PingPongTransactionChain is a very complicated piece of machinery, which has only 21.5% coverage (and has the most uncovered lines of all of MD-SAL) because most of the time we are hitting happy paths.

We do not want to disturb its layout or implementation, but as Mockito has support for mocking final classes, we should be able to better control its testing, covering unhappy paths as well.