Test Downloader backoff on http error codes, e.g. 503
When a server responds with a HTTP 503, Pulp downloaders are expected to backoff. For example consider a concurrency of 5, when downloading 20 things, the 5th download returns a HTTP 503 response.
Assuming the first 4 downloads don't complete during the backoff period, there will be a backoff period of a few seconds before any additional requests comes in because there can only be 5 and the 5th is in backoff
I recommend testing this using an aiohttp server implementation that serves with a concurrency of 6, returns a 503 on the 5th call, and asserts that a backoff delay is experienced.