Disable retry in tests

The client now has retry logic (https://github.com/aio-libs/aiohttp/blob/ba51537349a9acdf4b9d0427c4a21e0fb6a2ae05/aiohttp/client.py#L642), but this logic can mask issues in tests.
We should disable the retry logic by default in tests (e.g. when using TestClient).