Tests: Add stub for the internals object when not running in test mode

This allows developers to open LibWeb test html files in Ladybird or in
other browsers to observe their behavior without the internals object.

Tests/LibWeb/Text/input/include.js

@@ -1,5 +1,11 @@
 var __outputElement = null;
 
+if (globalThis.internals === undefined) {
+    internals = {
+        signalTextTestIsDone: function () {},
+    };
+}
+
 function println(s) {
     __outputElement.appendChild(document.createTextNode(s + "\n"));
 }