Refactor system tests to reuse NiFi instances across tests, where applicable

The System Tests are written in such a way that every new test class creates a new NiFi instance (or cluster) at the start and tears it down at the end.

While this provides the most "safety" for running the tests, it's very expensive and leads to the system tests running very slowly. Where possible, we should reuse a NiFi instance/cluster across tests. This can significantly reduce the amount of time required to run system tests, especially on Github Actions