Fix worker prune test to actually run its assertion & cover reality.

The block on Resque::Worker#work gets called *after* a job performs,
but nothing was in the queue so it never got evaluated.

    # https://github.com/resque/resque/blob/74c2025ab5fa46cdcbc4e13c4e3d044d46443fa0/test/worker_test.rb#L464-L466
    @worker.work(0) do
      assert_equal 1, Resque.workers.size
    end

This commit provides coverage for reality, ensuring that we prune
workers that:

1. track the exact same queues as us
2. are on the current host
3. not currently running