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.idl

@@ -39,4 +39,5 @@ interface Internals {
     undefined expireCookiesWithTimeOffset(long long seconds);
 
     DOMString getComputedLabel(Element element);
+    unsigned short getEchoServerPort();
 };