[nexus] fix flaky test 1_2_MATN_TC_10 by increasing wait time (openthread#13007)

This commit addresses an occasional failure in test 1_2_MATN_TC_10
where the Router's ping reply was not found in Step 8.

- Increased the time advanced in Step 8 from 10 seconds
  (kStabilizationTime) to 20 seconds (2 * kStabilizationTime).
- This allows more time for address resolution (NS/NA) and packet
  transmission in the simulated environment.
- Verified that the test passes consistently with 100 consecutive
  successful runs after applying this fix.

Author: jwhui

tests/nexus/test_1_2_MATN_TC_10.cpp

@@ -49,6 +49,11 @@ static constexpr uint32_t kAttachToRouterTime = 200 * 1000;
  */
 static constexpr uint32_t kStabilizationTime = 10 * 1000;
 
+/**
+ * Time to advance for address resolution and ping reply, in milliseconds.
+ */
+static constexpr uint32_t kAddressResolutionTime = 20 * 1000;
+
 /**
  * Time to advance for the BBR selection to complete, in milliseconds.
  */
@@ -298,7 +303,7 @@ void TestMatnTc10(void)
      * - Pass Criteria:
      *   - N/A
      */
-    nexus.AdvanceTime(kStabilizationTime);
+    nexus.AdvanceTime(kAddressResolutionTime);
 
     Log("---------------------------------------------------------------------------------------");
     Log("Step 8a: BR_1 powers down");