HTML: Add test for document.open() and MutationObserver (#12879)

For whatwg/html#3818.

html/webappapis/dynamic-markup-insertion/opening-the-input-stream/mutation-observer.window.js

@@ -0,0 +1,19 @@
+async_test(t => {
+  const frame = document.body.appendChild(document.createElement("iframe"));
+  t.add_cleanup(() => { frame.remove(); });
+  const originalHTMLElement = frame.contentDocument.documentElement;
+  assert_equals(originalHTMLElement.localName, "html");
+  const observer = new frame.contentWindow.MutationObserver(t.step_func_done(records => {
+    // Even though we passed `subtree: true` to observer.observe, due to the
+    // fact that "replace all" algorithm removes children with the "suppress
+    // observers flag" set, we still only get the html element as the sole
+    // removed node.
+    assert_equals(records.length, 1);
+    assert_equals(records[0].type, "childList");
+    assert_equals(records[0].target, frame.contentDocument);
+    assert_array_equals(records[0].addedNodes, []);
+    assert_array_equals(records[0].removedNodes, [originalHTMLElement]);
+  }));
+  observer.observe(frame.contentDocument, { childList: true, subtree: true });
+  assert_equals(frame.contentDocument.open(), frame.contentDocument);
+}, "document.open() should inform mutation observer of node removal");