feat(aggregator): Add additional unit tests for aggregator retries

[PatStiles]

Currently the unit tests for the aggregator retries only test the returned value and effects of all retries being provided. We want to also validate the intermediate states that can occur over multiple retries. To do so we should first separate the external logic of each Retryable from the retry library by declaring them as separate functions for testing purposes and create tests for these functions following a similar pattern to the current unit tests.