Streamline `EventProcessor` tests

[smcvb]

Description

One of the larger components that are tested in Axon Framework are the EventProcessor implementations. For "newer" infrastructure components, like the DeadlineManager and DeadLetterQueue, we went the route to construct a test suite that could be reused by each implementation. This flow is currently not in place for our Event Processors, although they would benefit greatly from this. Hence, the intent of this issue is to make such an event processor test suite.

Note

It would not be reasonable to start working on this before the overall event handling process has been revised.