cmd/devp2p: add discovery endpoint tests

[flcl42]

Summary

• Add a discv4 protocol test that bonds a synthetic peer with distinct UDP and TCP ports, then verifies FINDNODE/NEIGHBORS preserves both endpoint ports.
• Extend the discv5 FINDNODE zero-distance test to validate the returned self ENR endpoint entries are usable and internally paired.
• Add optional devp2p discv5 test flags for expected IPv4 and IPv6 self-record endpoint addresses so external harnesses can validate configured dual-stack advertisements.
• Make the discv5 test listener use net.JoinHostPort so IPv6 listen addresses work.

Validation

• go test ./cmd/devp2p/...
• go test ./p2p/discover/... ./p2p/enode/... ./p2p/enr/...
• go build ./cmd/devp2p
• git diff --check

Related

• Hive integration PR: simulators/devp2p: add dual-stack discovery orchestration hive#1598