[Feature Request] Allow mocking child workflows like activites

[KorDum]

Is your feature request related to a problem? Please describe.

In a situation where one workflow starts a child workflow, there is no way to replace the child workflow with a stub. Or I don't understand how to do it - there is no word about it in the documentation.

I found some strange behavior that led me to these thoughts.
Let's imagine that we have two workflows:

1. Workflow A
2. Workflow B, which is run via asyncDetached in Workflow A. Workflow B has a timer for 30 minutes

This tricky setup of two workflows allows to detect a bug (as I think). If we run an integration test on Workflow A where only the activity are replaced with stubs, then run an integration test on Workflow B, the timer waits an honest 30 minutes.

If we run only the test on Workflow B, the timer will run instantly without any waiting.

Describe the solution you'd like

It seems that if you replace Workflow B in the test for Workflow A with a stub, this strange problem goes away.