net: Use mockable time for ping/pong, add tests

test/functional/test_runner.py

@@ -236,6 +236,7 @@ BASE_SCRIPTS = [
     'mempool_compatibility.py',
     'rpc_deriveaddresses.py',
     'rpc_deriveaddresses.py --usecli',
+    'p2p_ping.py',
     'rpc_scantxoutset.py',
     'feature_logging.py',
     'p2p_node_network_limited.py',