refactor: migrate current `bentoml.testing` to use `bentoml.client`

[aarnphm]

Feature request

With https://github.com/bentoml/BentoML/pull/3028 it introduces bentoml.client.

Currently what we are doing under bentoml.testing are essentially doing what bentoml.client is doing.

This refactor would make bentoml.testing more clean and dry ;)

Motivation

No response

Other

No response