Fix test_client fixture to allow multiple clients per test

[popravich]

What do these changes do?

test_client fixture now allows multiple client instances:

from project import frontend, api

async def test_full_app(test_client):
    front_client = await test_client(frontend.make_app)
    api_client = await test_client(api.make_app)
    # test complete application

[codecov-io]

Current coverage is 97.48% (diff: 100%)

Merging #1072 into master will increase coverage by <.01%

@@             master      #1072   diff @@
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  Files            28         28          
  Lines          6532       6533     +1   
  Methods           0          0          
  Messages          0          0          
  Branches       1101       1101          
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+ Hits           6368       6369     +1   
  Misses           88         88          
  Partials         76         76          

Powered by Codecov. Last update b0231dc...8e5d508

[asvetlov]

Not bad but what's you use case?
I'm just curious.

[popravich]

Use case is almost as in example above:
project has two servers - public and internal APIs.
I'm testing that touching private API properly propagates data to public API.

[asvetlov]

lgtm

[lock]

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