Make TestCluster based integration tests more repoducible

While testing an async system providing reproducible tests that
use randomized components is a hard task we should at least try to
reestablish the enviroment of a failing test as much as possible.
This commit allows to re-establish the shared 'TestCluster' by
resetting the cluster to a predefined shared state before each test.

Before this commit a tests that is executed in isolation was likely
using a entirely different node enviroment as the failing test since
the 'TestCluster' kept intermediate nodes started by other tests around.