[routing-manager] enhance TestPrefixStaleTime() (openthread#10342)

This commit changes unit test `TestPrefixStaleTime()` so that a route
prefix is advertised with a lifetime of 800 seconds, which is longer
than the `kStaleTime` of 600 seconds. With this change, the entry
will not expire and become stale at the same time. This simultaneous
expiration could cause issues depending on the order of execution of
`mStaleTimer` and `mExpirationTimer`, which would be scheduled for
the same time. With this change, the entry will still be valid (due
to the lifetime of 800 seconds) when the stale timer fires after 600
seconds. This ensures that the test runs consistently and does not
depend on the order in which timer callbacks fire.

tests/unit/test_routing_manager.cpp

@@ -2692,11 +2692,11 @@ void TestPrefixStaleTime(void)
 
     //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     // Advertise a route prefix with 200 seconds lifetime from router A.
-    // Advertise the same route prefix with 500 seconds lifetime from
+    // Advertise the same route prefix with 800 seconds lifetime from
     // router B.
 
     SendRouterAdvert(routerAddressA, {Rio(routePrefix, 200, NetworkData::kRoutePreferenceMedium)});
-    SendRouterAdvert(routerAddressB, {Rio(routePrefix, 500, NetworkData::kRoutePreferenceMedium)});
+    SendRouterAdvert(routerAddressB, {Rio(routePrefix, 800, NetworkData::kRoutePreferenceMedium)});
 
     AdvanceTime(10);
 
@@ -2705,7 +2705,7 @@ void TestPrefixStaleTime(void)
     // is present in the table.
 
     VerifyPrefixTable({RoutePrefix(routePrefix, 200, NetworkData::kRoutePreferenceMedium, routerAddressA),
-                       RoutePrefix(routePrefix, 500, NetworkData::kRoutePreferenceMedium, routerAddressB)});
+                       RoutePrefix(routePrefix, 800, NetworkData::kRoutePreferenceMedium, routerAddressB)});
 
     //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     // Wait for a period exceeding the 200-second lifetime of the route
@@ -2716,19 +2716,21 @@ void TestPrefixStaleTime(void)
 
     sRsEmitted = false;
 
-    AdvanceTime(490 * 1000);
+    AdvanceTime(590 * 1000);
 
     VerifyOrQuit(!sRsEmitted);
 
     //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     // Check the discovered prefix table and ensure router A entry is
     // expired and removed.
 
-    VerifyPrefixTable({RoutePrefix(routePrefix, 500, NetworkData::kRoutePreferenceMedium, routerAddressB)});
+    VerifyPrefixTable({RoutePrefix(routePrefix, 800, NetworkData::kRoutePreferenceMedium, routerAddressB)});
 
     //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-    // Wait longer than 500-second lifetime of prefix advertised by
-    // router B. Now we should see RS messages emitted.
+    // Wait for the 600-second stale time to expire. This is shorter
+    // than the 800-second lifetime of the prefix advertised by
+    // Router B, so the 600-second value will be used. We should now
+    // observe RS messages being transmitted.
 
     AdvanceTime(20 * 1000);