Type has been exposed only for testing purposes

[Type here](https://github.com/ustulation/p2p/blob/c7f99c79bd1ef2ea4bd844acd4057ad55a280b6e/src/udp/hole_punch/mod.rs#L2) has been exposed only for testing purpose. See if there's a better way to test to not have to expose it or expose it only for non-production as exposure conveys wrong information to a reader.