[TD-4812] monitor dnode and mnode status and save to log

tests/robust/monitor.sh

@@ -0,0 +1,8 @@
+while :
+do
+    dlog=`taos -s "show dnodes"`
+    mlog=`taos -s "show mnodes"`
+    echo "$dlog" | tee -a dnode.log
+    echo "$mlog" | tee -a mnode.log
+    sleep 1s
+done