Add action to wait for process instance to be created

[pihme]

Is your feature request related to a problem? Please describe.
Currently I can only interact with process instances that were created by the test itself. I do not know about process instances that were created by the engine (e.g. event sub-processes, call activities).
What is needed is a way to:

• wait until a process instance of a given ID has been created
• assign some kind of (indexed) alias to identify the created process

Describe the solution you'd like
Something along those lines:

     - action: await-instance
        args:
          bpmn_process_id: sub-process
          workflow_instance_alias: C

This still doesn't address how to identify the third process instance being created, but it would be a step forward.

Describe alternatives you've considered

• Add a service task at the begin of a subprocess. The job activation would be a proxy for the workflow creation. However, I am not aware of any way to get the process Id and feed it back to the test enigne; also it would be not clear at all in the test spec what is going on
• Some way of using an action to query the tree of nested processes and then based on the query result move forward; didn't really make much sense, to be honest.