Ruby how to test thread : How to check the result of some method inside thread.

For example:

we have thread:

    module Task
     def self.execute
      "result"
     end
    end

    threads = []
    threads << Thread.new do
      Task.execute
    end

Now we need to specify the test which check the result:
expect(Task.execute).to eq("result")

Ok. Let's try to add thread inside thread:

    threads << Thread.new do
      deep_thread = Thread.new { Task.execute }
      deep_thread.join
    end

So, how to check that the fact of two finished threads, and also check the result of `deep_thread` ?