feature: provide some sort of mock "EventOps" for unit-testing

[acatangiu]

The fn process(&self, events: Events, ops: &mut EventOps) function of trait (Mut)EventSubscriber contains the core event-handling logic of any user or this crate.

It would be nice to be able to unit-test it and cover that critical functionality.

Currently, the EventOps reference prevents any outside/test calls into process() because EventOps doesn't have any public constructors, and such objects are only built by event-manager crate-internal code.

I don't have a clear idea right now on what would be the best way to allow unit-testing process(); the real EventOps object also contains other internal/non-public members so we can't just stick a public constructor and be done with it...

Any ideas? 😃