Auto merge of #17480 - asajeffrey:wpt-reenable-iframe-append-to-child-document, r=emilio

Re-enabled iframe-append-to-child-document.html

<!-- Please describe your changes on the following line: -->

Re-enabled a WPT test. Don't get too excited, the test FAILs.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15196
- [X] These changes do not require tests because it's re-enabling a test.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17480)
<!-- Reviewable:end -->

tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe-append-to-child-document.html.ini

@@ -1,3 +1,5 @@
 [iframe-append-to-child-document.html]
   type: testharness
-  disabled: https://github.com/servo/servo/issues/14411
+  [Append iframe element to its own child document]
+    bug: https://github.com/servo/servo/issues/17479
+    expected: FAIL