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/HTML/document.head.js

@@ -0,0 +1,16 @@
+loadPage("file:///res/html/misc/blank.html");
+
+afterInitialPageLoad(() => {
+    test("Basic functionality", () => {
+        expect(document.head).not.toBeNull();
+        // FIXME: Add this in once HTMLHeadElement's constructor is implemented.
+        //expect(document.head).toBeInstanceOf(HTMLHeadElement);
+        expect(document.head.nodeName).toBe("head");
+    });
+
+    // FIXME: Add this in once removeChild is implemented.
+    test.skip("Nullable", () => {
+        document.documentElement.removeChild(document.head);
+        expect(document.head).toBeNull();
+    });
+});