I realize that you are emulating a pattern found elsewhere in this test, so feel free to ignore this suggestion, but I think this is kind of goofy. I think it's a little less goofy if you have the worker instance extend a module that overrides the default behavior. It also helps DRY the code in case the override should need to be changed later. Also, using yield instead of block.call saves a few characters:
```
worker.extend(OverrideCreateThread)
```
Then at the end of the file (or in test_helper if this pattern is applicable across multiple tests):
```
module OverrideCreateThread
  def create_thread(*args, &block)
      yield(*args)
  end
end
```

I have verified that the suggested pattern will work in all the other places the existing pattern is used.