net: add new method Sock::Listen() that wraps listen()

This will help to increase `Sock` usage and make more code mockable.

src/test/fuzz/util.h

@@ -66,6 +66,8 @@ public:
 
     int Bind(const sockaddr*, socklen_t) const override;
 
+    int Listen(int backlog) const override;
+
     std::unique_ptr<Sock> Accept(sockaddr* addr, socklen_t* addr_len) const override;
 
     int GetSockOpt(int level, int opt_name, void* opt_val, socklen_t* opt_len) const override;