NIFI-9233 - Improve reliability of system integration tests

[greyp9]

Description of PR

Avoid deadlock on process shutdown during system test invocation. This SaveReportingTask lock usage contends with lock in StandardFlowService.stop() (which releases after gracefulShutdownSeconds).

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[markap14]

Thanks for the update @greyp9. This makes sense, avoids an issue in the system tests where we update the flow and immediately shutdown before changes are saved. +1 merged to main.