LibJS: Convert all remaining non-Array tests to the new system :)

Libraries/LibJS/Tests/classes/class-advanced-extends.js

@@ -0,0 +1,35 @@
+test("extending function", () => {
+  class A extends function () {
+    this.foo = 10;
+  } {}
+
+  expect(new A().foo).toBe(10);
+});
+
+test("extending null", () => {
+  class A extends null {}
+
+  expect(Object.getPrototypeOf(A.prototype)).toBeNull();
+
+  expect(() => {
+    new A();
+  }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+});
+
+test("extending String", () => {
+  class MyString extends String {}
+
+  const ms = new MyString("abc");
+  expect(ms).toBeInstanceOf(MyString);
+  expect(ms).toBeInstanceOf(String);
+  expect(ms.charAt(1)).toBe("b");
+
+  class MyString2 extends MyString {
+    charAt(i) {
+      return `#${super.charAt(i)}`;
+    }
+  }
+
+  const ms2 = new MyString2("abc");
+  expect(ms2.charAt(1)).toBe("#b");
+});