LibWeb+Tests: Add internal property for echo server port

Author: https://github.com/rmg-x
Commit: cddbe7d10f
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2553
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/shannonbooth

Libraries/LibWeb/Internals/Internals.h

@@ -50,6 +50,9 @@ public:
 
     String get_computed_label(DOM::Element& element);
 
+    static u16 get_echo_server_port();
+    static void set_echo_server_port(u16 port);
+
 private:
     explicit Internals(JS::Realm&);
     virtual void initialize(JS::Realm&) override;