LibWeb: Add more document tests, add comment, text and mixin tests

Also adds a TypeScript definition file for the test runner object.

Libraries/LibWeb/Tests/libweb_tester.d.ts

@@ -0,0 +1,16 @@
+// NOTE: This file is only for syntax highlighting, documentation, etc.
+
+interface LibwebTester {
+    /**
+     * Changes the page to the specified URL. Everything afterwards will refer to the new page.
+     * @param url Page to load.
+     */
+    changePage(url: string): void;
+}
+
+interface Window {
+    /**
+     * Special test object used to ease test development for LibWeb.
+     */
+    readonly libweb_tester: LibwebTester;
+}