Fix test review issues

- Remove wrong latest_message assertions from payload attestation test
  (on_payload_attestation writes to PTC bitfields, not vote tracker)
- Fix corrupted comment: "votes.gloas_enabled() to the genesis block"
  → "votes to the genesis block"
- Fix http_api test fallback string: "n/a" → "irrelevant" to match
  production code
- Add issue link to #[ignore] test
- Add comment explaining head_payload_status as u8 cast

beacon_node/http_api/tests/tests.rs

@@ -3176,7 +3176,7 @@ impl ApiTester {
                             .execution_status()
                             .ok()
                             .map(|status| status.to_string())
-                            .unwrap_or_else(|| "n/a".to_string()),
+                            .unwrap_or_else(|| "irrelevant".to_string()),
                         best_child: node
                             .best_child()
                             .and_then(|index| expected_proto_array.nodes.get(index))