Fix an unused parameter warning in dhtrunnertester

This change eliminates the following compiler warning without reducing readability:

/home/noviv/opendht/tests/dhtrunnertester.cpp: In lambda function:
/home/noviv/opendht/tests/dhtrunnertester.cpp:292:89: warning: unused parameter ‘expired’ [-Wunused-parameter]
  292 |     node2.listen(key2, [&](const std::vector<std::shared_ptr<dht::Value>>& values, bool expired){
      |                                                                                    ~~~~~^~~~~~~