[Test] Chaining sub-workflows

[fmigneault]

Technically it would be possible to chain sub-workflows as step of a bigger workflow.
This has to be tested though in the context of weaver.

There is a good chance that data-source resolution (where to dispatch the step process) fails since weaver looks for a remote ADES to run the process. Since in that case the step would be a workflow, it needs to be executed by an EMS.
There is also a change that weaver auto-resolves the default data-source to 'self' and it could then transparently work, but it is not a guarantee.

We should probably consider some package type check that enforce resolution of a sub-workflow to 'self' to avoid unexpected problems.

It is also important to resolve to 'self' instead of "any remote EMS" because the sub-workflow step will need to find all sub-sub-processes to call them. Dispatching a sub-workflow step to another EMS would require to recursively dispatch every underlying process application, which rapidly becomes unmanageable. Also, there is technically no advantage to deploy to a remote EMS since its job should be only to monitor remote jobs, which should be low in terms of resource requirements.