test: cover timing, overflow, and target authentication contracts

crates/irtt-client/tests/linux_transport.rs

@@ -132,9 +132,14 @@ fn assert_kernel_send_endpoint(event: &ClientEvent, before_send: SystemTime) {
         "TX timestamp was not observed; userspace fallback is insufficient"
     );
     assert!(selected <= ns(received_at.wall));
-    assert!(
-        packet_meta.kernel_rx_timestamp.is_some(),
-        "Linux RX timestamp required"
+    let selected_receive =
+        i128::from(timing.send_wall_ns.unwrap()) + one_way.server_to_client.unwrap().as_nanos();
+    assert_eq!(
+        selected_receive,
+        ns(packet_meta
+            .kernel_rx_timestamp
+            .expect("Linux RX timestamp required")),
+        "downstream OWD must use the kernel receive endpoint"
     );
     assert_eq!(rtt.raw, received_at.mono.duration_since(sent_at.mono));
 }