[ConcurrentNetworkEchoTest] reduce echos to prevent timeouts on slower CI nodes

tests/io/ConcurrentNetworkEchoTest.cpp

@@ -22,7 +22,7 @@ auto main(int argc, char* argv[]) -> int {
 	}
 
 	unsigned int client_count = 10;
-	unsigned int echos = 1000;
+	unsigned int echos = 500;
 	std::string port = "4241";
 	std::string host = "127.0.0.1";