[Docs] Manual TestClient creation should be async

[ErwanDL]

🐞 Describe the bug
In the Framework agnostic utilities section of the "Testing" docs, this code snippet indicates how to manually create a TestClient :

with loop_context() as loop:
    app = _create_example_app()
    with TestClient(TestServer(app), loop=loop) as client:

        async def test_get_route():
            nonlocal client
            resp = await client.get("/")
            assert resp.status == 200
            text = await resp.text()
            assert "Hello, world" in text

        loop.run_until_complete(test_get_route())

However, when trying to create a TestClient that way, you get the following error :

TypeError: Use async with instead
Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x7fc6daa51a60>

I suspect the docs should rather indicate that the TestClient be created that way :

async with TestClient(TestServer(app), loop=loop) as client:
    ...

[asvetlov]

Correct.
I also want to get rid of explicit loops in examples