Fix testing with DI in Workflow

[xlegalles]

Dependency injection is supported in Workflow but cannot be tested (fix)
It is related to changes made in PR: Allow dependency injection on Workflow #576
The new() constraint on TWokflow is still present on WorkflowTest while it has been removed elsewhere

[xlegalles]

It may also be useful to add comments in the documentation:

• "step and workflow classes must be registered in the DI container if they expect dependencies to be injected". This is obvious, but personally I was expecting that it was already done by default.
• "step classes must inherit directly from StepBody/StepBodyAsync i.e. cannot inherit from intermediate classes". No factorization allowed as I've started to do.

Thank you

[daniele-betsson]

would be fantastic to have this merged some years later :D