HTML: test WindowProxy self references

For whatwg/html#4410.

html/browsers/the-window-object/self-et-al.window.js

@@ -0,0 +1,26 @@
+[
+  "frames",
+  "globalThis",
+  "self",
+  "window"
+].forEach(windowProxySelfReference => {
+  test(() => {
+    const frame = document.body.appendChild(document.createElement("iframe")),
+          otherW = frame.contentWindow;
+    assert_equals(otherW[windowProxySelfReference], otherW);
+    frame.remove();
+    assert_equals(otherW[windowProxySelfReference], otherW);
+  }, `iframeWindow.${windowProxySelfReference} before and after removal`);
+
+  async_test(t => {
+    const otherW = window.open();
+    assert_equals(otherW[windowProxySelfReference], otherW);
+    otherW.close();
+    assert_equals(otherW[windowProxySelfReference], otherW);
+    t.step_timeout(() => {
+      assert_equals(otherW.opener, null); // Ensure browsing context is discarded
+      assert_equals(otherW[windowProxySelfReference], otherW);
+      t.done();
+    }, 250);
+  }, `popupWindow.${windowProxySelfReference} before, after closing, and after discarding`)
+});