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?