Extend the servertest harness with:
- TestClient.Direct() accessor for advanced operations
- TestClient.WaitForPeerCount and WaitForCondition helpers
- TestHarness.ChangePolicy for ACL policy testing
- AssertDERPMapPresent and AssertSelfHasAddresses
New test suites:
- content_test.go: self node, DERP map, peer properties, user profiles,
update history monotonicity, and endpoint update propagation
- policy_test.go: default allow-all, explicit policy, policy triggers
updates on all nodes, multiple policy changes, multi-user mesh
- ephemeral_test.go: ephemeral connect, cleanup after disconnect,
mixed ephemeral/regular, reconnect prevents cleanup
- routes_test.go: addresses in AllowedIPs, route advertise and approve,
advertised routes via hostinfo, CGNAT range validation
Also fix node_departs test to use WaitForCondition instead of
assert.Eventually, and convert concurrent_join_and_leave to
interleaved_join_and_leave with grace-period-tolerant assertions.
