RedisFeederTest should shut down GatlingActorSystem after test

Refactored `RedisFeederTest` to shutdown `GatlingActorSystem` after use - otherwise, there are sporadic test failures for other tests that use `GatlingActorSystem`.

I re-used the `ActorSupport` functionality used elsewhere, and rejigged it to initialise `GatlingConfiguration`, as `RedisFeederTest` expects.