Add response sender test base

[kthui]

Set up the base for response sender testing, which enables testing varies ways a model may return response(s), including improper use of response sender.
Currently, this test only tests existing behaviors of decoupled and non-decoupled models, before any work is done on their data pipelines or response sender.

The control of testing is from the test script by setting the input tensors:

• number_of_response_before_return
• send_complete_final_flag_before_return
• return_a_response
• number_of_response_after_return
• send_complete_final_flag_after_return

There are definite some improvements possible on the test base, i.e. reporting the size of requests, allow the final flag to be sent with the last response or separately, add support for dynamic batching requests. but I think those can be added (or already added) in next PRs when needed.

Next PRs:

• Add response sender test for new behavior #7257
• Add response sender to non-decoupled models and unify data pipelines python_backend#360

[rmccorm4]

High level overview of tests look good to me, left some comments.