ut_routing_table: fix testMobileNodeAnnouncement

shutdown is not immediate, so the assert was failing. Waiting a bit
that the shutdown finishes fix the test.

Change-Id: Iee88eecd6a78f9115ac057b6321fb032d5feb911

test/unitTest/swarm/routing_table.cpp

@@ -898,6 +898,7 @@ RoutingTableTest::testMobileNodeAnnouncement()
         rt1.hasNode(NodeId("41a05179e4b3e42c3409b10280bb448d5bbd5ef64784b997d2d1663457bb6ba8")));
 
     sm2->shutdown();
+    sleep(5);
 
     auto mb1 = rt1.getMobileNodes();