Add response sender test for new behavior

[kthui]

Previous PR: #7254
Related PR: triton-inference-server/python_backend#360

Add the following new tests:

• Non-decoupled model sending a response before / after returning.
• Decoupled / non-decoupled model handling batched requests each using a different style of returning response(s).
• Assert failure on decoupled model returning a response.

Next PR: triton-inference-server/python_backend#361

[kthui]

Rebasing onto https://github.com/triton-inference-server/server/tree/jacky-res-sender-main for merging.