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.body.js

@@ -0,0 +1,55 @@
+loadPage("file:///res/html/misc/blank.html");
+
+afterInitialPageLoad(() => {
+    test("Basic functionality", () => {
+        expect(document.body).not.toBeNull();
+        // FIXME: Add this in once HTMLBodyElement's constructor is implemented.
+        //expect(document.body).toBeInstanceOf(HTMLBodyElement);
+        expect(document.body.nodeName).toBe("body");
+    });
+
+    // FIXME: Add this in once set_body is fully implemented.
+    test.skip("Setting body to a new body element", () => {
+        // Add something to body to see if it's gone afterwards
+        const p = document.createElement("p");
+        document.body.appendChild(p);
+
+        expect(document.body.firstChild).toBe(p);
+
+        const newBody = document.createElement("body");
+        document.body = newBody;
+
+        expect(document.body).not.toBeNull();
+        expect(document.body.nodeName).toBe("body");
+
+        // FIXME: Add this in once HTMLBodyElement's constructor is implemented.
+        //expect(document.body).toBeInstanceOf(HTMLBodyElement);
+
+        expect(document.body.firstChild).toBeNull();
+    });
+
+    // FIXME: Add this in once set_body is fully implemented.
+    test.skip("Setting body to a new frameset element", () => {
+        const newFrameSet = document.createElement("frameset");
+        document.body = newFrameSet;
+
+        expect(document.body).not.toBeNull();
+        expect(document.body.nodeName).toBe("frameset");
+
+        // FIXME: Add this in once HTMLFrameSetElement's constructor is implemented.
+        //expect(document.body).toBeInstanceOf(HTMLFrameSetElement);
+    });
+
+    // FIXME: Add this in once set_body is fully implemented.
+    test.skip("Setting body to an element that isn't body/frameset", () => {
+        expect(() => {
+            document.body = document.createElement("div");
+        }).toThrow(DOMException);
+    });
+
+    // FIXME: Add this in once removeChild is implemented.
+    test.skip("Nullable", () => {
+       document.documentElement.removeChild(document.body);
+       expect(document.body).toBeNull();
+    });
+});