Replace rand() with fixed-seed RNG to make tests deterministic

[alienx5499]

Summary

Replaces unseeded rand() in tests with a shared fixed-seed RNG to make test runs deterministic and reproducible, preventing flaky CI failures.

Why

Unseeded randomness causes flaky tests and non-reproducible CI failures.

What changed

• Added testutil::randomBytes() and getTestRng() in test/testutil/libp2p/random.hpp.
• Replaced rand() usage in:
  • test/testutil/libp2p/peer.cpp
  • test/libp2p/transport/tcp/tcp_integration_test.cpp
  • test/libp2p/crypto/marshaller_test.cpp
  • test/libp2p/protocol/kademlia/peer_routing_table_test.cpp

Impact

• No functional changes.
• Improves CI stability and debugging reliability.

Note

The RNG is shared (static); test execution order affects the byte sequence. Determinism is per run, not per test. See comment in random.hpp.