Refactor unit test templates

[atulep]

Currently, unit tests in templates code are all concentrated in a single file test_%service.py.j2. The file has 3K lines and should be refactored.

One idea is to refactor across two dimensions: transport and sync/async (cc @software-dov). It's a big effort, so perhaps a smaller task is to first identify the common boilerplate code in the existing test file and lift it to a separate file.