NIFI-5824: Added unit test to FlowController to ensure that the Proce…

[markap14]

…ssScheduler that it creates is properly initialized. Also updated the properties file used by TestFlowController to use a VolatileContentRepository instead of FileSystemRepository, and fixed EventDrivenWorkerQueue to return if calls to poll() are interrupted (via Thread.interrupt) - making these minor fixes resulted in the unit test TestFlowController running in 2 seconds instead of 30+ seconds on my machine

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[bbende]

Code looks good and tests pass, will merge