[nexus] fix intermittent failures in test 7.1.8 (#12543)

This commit fixes occasional failures in Nexus test 7.1.8 by making
the verification script more robust against interleaved packets.

The implementation details:
- tests/nexus/verify_7_1_8.py: Refactored Step 3 to use separate
  filter copies to find the Leader's MLE Data Response and CoAP ACK
  independently of their order.
- Correctly updated the main packet filter index to ensure that
  subsequent verification steps proceed from the point after both
  packets have been identified.

tests/nexus/verify_7_1_8.py

@@ -101,17 +101,25 @@ def verify(pv):
     #   on FED_1.
     # - Pass Criteria: N/A.
     print("Step 3: Leader transmits a 2.04 Changed CoAP response and multicast MLE Data Response.")
-    pkts.filter_wpan_src64(LEADER).\
-      filter_LLANMA().\
-      filter_mle_cmd(consts.MLE_DATA_RESPONSE).\
-      filter(lambda p: {
-        PREFIX_1,
-        PREFIX_2
-      } <= set(p.thread_nwd.tlv.prefix)).\
-      must_next()
-    pkts.filter_wpan_src64(LEADER).\
-      filter_coap_ack(consts.SVR_DATA_URI).\
-      must_next()
+    # We use two separate filters to find MLE Data Response and CoAP ACK independently,
+    # as their order is not guaranteed.
+    mle_filter = pkts.copy().\
+        filter_wpan_src64(LEADER).\
+        filter_LLANMA().\
+        filter_mle_cmd(consts.MLE_DATA_RESPONSE).\
+        filter(lambda p: {
+            PREFIX_1,
+            PREFIX_2
+        } <= set(p.thread_nwd.tlv.prefix))
+    mle_filter.must_next()
+
+    coap_filter = pkts.copy().\
+        filter_wpan_src64(LEADER).\
+        filter_coap_ack(consts.SVR_DATA_URI)
+    coap_filter.must_next()
+
+    # Advance the main filter's index past both packets.
+    pkts.index = (max(mle_filter.index[0], coap_filter.index[0]), max(mle_filter.index[1], coap_filter.index[1]))
 
     # Step 4: Router_1 (DUT)
     # - Description: Automatically transmits multicast MLE Data Response with the new information collected, adding
@@ -119,13 +127,13 @@ def verify(pv):
     # - Pass Criteria: The DUT MUST send a multicast MLE Data Response.
     print("Step 4: Router_1 (DUT) transmits multicast MLE Data Response.")
     pkts.filter_wpan_src64(ROUTER_1).\
-      filter_LLANMA().\
-      filter_mle_cmd(consts.MLE_DATA_RESPONSE).\
-      filter(lambda p: {
-        PREFIX_1,
-        PREFIX_2
-      } <= set(p.thread_nwd.tlv.prefix)).\
-      must_next()
+        filter_LLANMA().\
+        filter_mle_cmd(consts.MLE_DATA_RESPONSE).\
+        filter(lambda p: {
+            PREFIX_1,
+            PREFIX_2
+        } <= set(p.thread_nwd.tlv.prefix)).\
+        must_next()
 
     # Step 5: FED_1
     # - Description: Harness silently powers-down FED_1 and waits for Router_1 to remove FED_1 from its neighbor