Address review feedback on the peer add/done race fix:
Add three direct unit tests in server_test.go that exercise the fix
without the full server or rpctest harness:
- TestOnVerAckDoubleCall: call OnVerAck twice on the same serverPeer,
assert no panic and verAckCh remains closed.
- TestPeerLifecycleOrdering: verack before disconnect emits peerAdd
then peerDone in order.
- TestPeerLifecycleSimultaneousReady: both verAckCh and Peer.Done()
ready before the handler runs; assert peerDone always arrives and
peerAdd, if emitted, precedes it (100 iterations).
Harden integration tests in sync_race_test.go:
- Check fakePeerConn errors via require.NoError instead of discarding.
- Extract dialAndSendVersion helper for TestPreVerackDisconnect;
check all errors instead of silently continuing.
- Fix comment wording ("produces" -> "is expected to produce").
