Add Testing Utility for Swarm

We recently changed a big part of the supervision of quantum (add job scheduler library) to swarm if it is used in a cluster.

One thing that was missing for us is a testing utility to make sure that the new callbacks for swarm actually work.

I implemented a really easy helper: https://github.com/quantum-elixir/quantum-core/pull/369/files#diff-51021d33872c9243af9315e6b3809e77

Would you be willing so add something like that (but more complete) to the library?

Start by reviewing the linked Quantum PR implementation, then inspect Swarm's callback behavior and existing test setup. Done means adding a more complete testing utility that can verify the new Swarm callbacks work in a cluster.

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