test(kad): use virtual time to resolve flaky bootstrap test

[0xsamalt]

Description

This PR fixes intermittent CPU-lag flakiness in Kademlia's bootstrap.rs test suite.

The tests previously measured real-world timeouts using futures_timer::Delay and web_time::Instant, causing them to fail when CI servers inevitably overshot expectations.

By leveraging Tokio's test-util feature and explicitly wrapping our mocks for the tests, #[tokio::test(start_paused = true)] ensures the elapsed test time is completely deterministic and decoupled from wall-clock performance. Tests run instantly and pass 100/100 times in local looping.

cc @jxs @elenaf9

Notes & open questions

• This is purely an internal test infrastructure fix. Production paths are untouched because futures_timer::Delay is preserved under #[cfg(not(test))].
• Because this does not affect any public API or library behavior, a changelog entry was intentionally omitted.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (N/A - Test fix)
• I have added tests that prove my fix is effective or that my feature works (Modified existing flaky tests)
• A changelog entry has been made in the appropriate crates (N/A - Internal testing change only)

[mergify]

This pull request has merge conflicts. Could you please resolve them @0xsamalt? 🙏