We now have a base test case class for Predis (namely PredisTestCase)
grouping various commonly used utility methods shared by all of the
tests in the suite, greatly improving reusability.
The Predis\Transaction\MultiExecState class is actually quite dummy as
it does not work as a finite-state machine, but is used to track state
using flags. It basically replicates the same behaviour of when it was
part of the Predis\Transaction\MultiExec code and it will do for now.