@Override
        public void visit(TraceNode node) {
            if (node.isRoot()) return;
            if (node.payload() == null) return;
            trace.add(indent + node.payload().toString());
        }
Tests tracing scenarios where traces from multiple executions over the same query are involved.

@author bratseth
container-search/src/test/java/com/yahoo/search/searchchain/test/TraceTestCase.java