Core: disabled cloning sockets when testing config (ticket #2188). Since we anyway do not set SO_REUSEPORT when testing configuration (see ecb5cd305b06), trying to open additional sockets does not make much sense, as all these additional sockets are expected to result in EADDRINUSE errors from bind(). On the other hand, there are reports that trying to open these sockets takes significant time under load: total configuration testing time greater than 15s was observed in ticket #2188, compared to less than 1s without load. With this change, no additional sockets are opened during testing configuration.