feat!: Testnet api cleanup

[SeverinAlexB]

This PR fixes some rough edges of Testnet.

• Add Testnet::dht_builder() method that returns a DhtBuilder pre-configured with the testnet's bootstrap nodes and localhost bind address. This reduces the lines of code needed to write a pkarr client for testnet significantly.
• Change Testnet::new() and Testnet::new_unseeded() to bind to 127.0.0.1 (localhost) instead of 0.0.0.0, matching the builder's existing default
• Remove the now-redundant new_async and new_unseeded_async methods (they were identical to their sync counterparts)
• Remove outdated TODO comments about the bind address change
• Reduced seeded testnet bootstrap time by half

This is a BREAKING CHANGE.