Update testing.md with fixed code for pytest blacksheep testing

[sinnwise]

This problem seems to come across with 3.11, don't exactly know the circumstances in why/which this error gets reported.

test_test.py::test_create_and_get_todo FAILED                            [100%]
test_test.py:4 (test_create_and_get_todo)
test_client = <coroutine object test_client at 0x108f84520>

    @pytest.mark.asyncio
    async def test_create_and_get_todo(test_client: TestClient) -> None:
>       response = await test_client.get("/")
E       AttributeError: 'coroutine' object has no attribute 'get'

test_test.py:7: AttributeError

But with pytest_asyncio.fixture used instead of pytest.fixture it seems to work.

Edit:

[Example Repo](https://github.com/sinnwise/test_blacksheep_pytest

[RobertoPrevato]

Hi @sinnwise
Sorry for replying so late. Thank You for your contribution, I merge it now and publish and update to the docs site later today.