add orchestration processor idempotency test

[famarting]

adds a new unit test that reproduces a bug I'm troubleshooting

the handling of execution started event is not idempotent and it doesn't guarantee that the orchestrator is actually executed when retried

my expectation is that when ProcessWorkItem returns an error, it is expected that ProcessWorkItem can be retried in the future with the same WorkItem, then the source of problems is when the state passed inside of the WorkItem is the same as the state used for the first ProcessWorkItem invocation. In that case ProcessWorkItem doesn't have an idempotent behavior and in the second invocation doesn't call w.executor.ExecuteOrchestrator

So the sequence of events I would expect are:

• WorkItem is provided in NextWorkItem
• WorkItem is processed in ProcessWorkItem, it invokes executor.ExecuteOrchestrator but it fails and ProcessWorkItem returns an error
• Same WorkItem (with same state) is provided again in NextWorkItem
• WorkItem is processed in ProcessWorkItem , it SHOULD invoke executor.ExecuteOrchestrator