net: convert standalone IsSelectableSocket() to Sock::IsSelectable()

This makes the callers mockable.

src/test/util/net.h

@@ -166,6 +166,8 @@ public:
         return 0;
     }
 
+    bool IsSelectable() const override { return true; }
+
     bool Wait(std::chrono::milliseconds timeout,
               Event requested,
               Event* occurred = nullptr) const override