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.createTextNode.js

@@ -0,0 +1,13 @@
+loadPage("file:///res/html/misc/blank.html");
+
+afterInitialPageLoad(() => {
+    test("Basic functionality", () => {
+        const text = document.createTextNode("Create Text Test");
+
+        // FIXME: Add this in once Text's constructor is implemented.
+        //expect(text).toBeInstanceOf(Text);
+        expect(text.nodeName).toBe("#text");
+        expect(text.data).toBe("Create Text Test");
+        expect(text.length).toBe(16);
+    });
+});