net: add new method Sock::SetSockOpt() that wraps setsockopt()

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

src/test/util/net.h

@@ -150,6 +150,8 @@ public:
         return 0;
     }
 
+    int SetSockOpt(int, int, const void*, socklen_t) const override { return 0; }
+
     bool Wait(std::chrono::milliseconds timeout,
               Event requested,
               Event* occurred = nullptr) const override