Introduce testing utilities for dazl

[da-tanabe]

dazl.connect_mock may actually be straightforward to implement now with the new dazl v8 API.

func exercise_handler(commands: ExerciseCommand) -> ExerciseResponse: ...

conn = dazl.connect_mock(act_as=..., read_as=..., exercise_fn=exercise_handler)
conn.upload_package(MY_DAR)
conn.submit_create('Some:Stuff', {...}) 

async with conn.query('Some:Stuff') as stream:
    async for ev in stream.creates():
        print(ev.contract_id, ev.payload)