Allow monitor to stabilize in mxs1516

The test needs to give the monitor enough time to detect the change in the
replication topology in order for it to work.

maxscale-system-test/mxs1516.cpp

@@ -17,6 +17,9 @@ int main(int argc, char** argv)
     test.repl->connect();
     test.repl->change_master(1, 0);
 
+    // Give the monitor some time to detect it
+    sleep(5);
+
     test.add_result(execute_query_silent(test.maxscales->conn_master[0], "SELECT 1") == 0, "Query should fail");
 
     // Change the master back to the original one