tests: validate seeking behavior and math

[Arqu]

    Looks good, please also open an issue to add some additional tests on the seeking math, that is easy to get wrong on edge cases and I don't trust my own review of them. Some nice proptests should help for confidence.

Originally posted by @dignifiedquire in https://github.com/n0-computer/iroh/pull/330#pullrequestreview-1142960272