Test retry of activity sending

[Nutomic]

There are many cases where delivery of a federated activity can fail, for example when the remote instance is temporarily offline. Lemmy has code to retry the delivery in this case, but we never tested if it actually works in practice, or if the parameters for exponential backoff are good.

Link to the code. For testing, it should help to add some extra logging in there, and try different values for retry and backoff.

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