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/DOM/document.createDocumentFragment.js

@@ -0,0 +1,11 @@
+loadPage("file:///res/html/misc/blank.html");
+
+afterInitialPageLoad(() => {
+    test("Basic functionality", () => {
+        const fragment = document.createDocumentFragment();
+
+        // FIXME: Add this in once DocumentFragment's constructor is implemented.
+        //expect(fragment).toBeInstanceOf(DocumentFragment);
+        expect(fragment.nodeName).toBe("#document-fragment");
+    });
+});