Results Topic Tests for KafkaExecutor

[JeVaughan]

There's been 2 issues found related to the PiEvents on the KafkaExecutor:

• Missing PiEventCalls
• Duplicate PiEventReturned

The following unit tests would be helpful:

• Always 1 PiEventStart
• At most 1 PiEventResult / exception per PiInstance ID.
• At most 1 PiEventCall per reference
• At most 1 PiEventReturn / exception per reference
• PiEventCall reference ids should be sequential